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ment, and hoping that much good may accrue from its complete or and thanks ; occasional queries of the kind he mentions may be addressed partial adoption by your students, I am, &c.-W. B.

to us.-SIMPLICITAS (Wemysa): His attempt at the solution of the Mathe

matical questions for matriculation in the U. of L., is very fair ; but a little Norwich, Feb. 3rd, 1864.

more perseverance would have been attended with greater success.-PHILOS [We have inserted this letter, because we think the plan a good name and address, and free us of expense in the matter.–J. WOOD (Man

(London): Any one who wishes us to recommend a tutor, must give his onc, being simple and free from objections which we have had to chester), and YOUNG MAN (Duntocher): We can't advise them.-D. R. B. some other plans which have been proposed. Of course the author (Dundee): Vol. II. of “Cassell's Classical Library” is finished.-SECULAhas given us privately his name and address, a measure quite RIST (Bp. Wearmouth) proposes the following question, which is very old, indispensable to any proposal of the same kind which we may it:-“An eight-gallon cask of water is to be divided equally between two

but ingenious. As it may be new to some of our young readers, we insert receive.]

persons who have no measure of any kind, but a three-gallon cask and a five-gallon cask. How is it to be done ?

MNEMONICS (Upper Stamford St.): We have never seen or heard of any

permanent good resulting from the application of Mnemonics to the acqui. ANSWERS TO CORRESPONDENTS.

sition of Languages or the Sciences. Ruddiman's old Latin example T.C. II. L.: We must refer you to previous remarks with regard to the

appears to us to be the best rule: Memoria excolendo augetur, that is, Italian Dictionary. The matter will be duly considered. With regard to the Mathematics.-W. MAUNDRELL (Seven-oaks): “Borrowed a sum s for no

" The memory is improved by exercising it.”—J.C. (Salisbury): Study the Key

in each number to the English-Italian Exercises of the previous week, it years at r per cent., to be paid off by the annual payment of a certain sum would facilitate the progress of some earnest pupils without the aid of a master, but at the same time further the idleness of by far the greatest Dr. Thomson, in his “ Arithmetic," p. 275, shows, that in order « to find

4. What ought that sum to be ?” Here, the sum u is an annuity. Now number. To satisfy their curiosity, they would compare the Key with the

the entire sum due for an annuity remaining unpaid for a given period, exercises of the previous week, and consider themselves exempted from the task of translating

them. Á living language is not learned, like a dead compound interest being allowed on all the money thus remaining unpaid," one, merely for the purpose of reading, but also for the purpose of speaking

(Q;,-1) and writing; and with regard to this latter most important object, the

we must use the formula 1 =

; where s, n, and u, have the self-activity of the pupils must be kept up by all means in the power of a teacher. However, a Key of all the previous English-Italian Exercises will

100+1 be given after the termination of each of the principal divisions of the

values given above, and a =

In order to solve the preceding grammar, and then we think without the above-stated disadvantage.

100 H. PITT (Birmingham): See our answer to T, K., Clerkenwell. The problem written out without reference to any diagram, is passably well. question, then, we have only to reverse the formula by a simple Algebraic Write to the Editor of the P. B. E. as you do to us.-J. D. (Birmingham):

(a,-1) 3 When the chord of an arc, and the radius of the circle are given, the chord

process,
thus: u =

Example: If the sum of £7,905 16s. 4fd. of half the arc may be found without Trigonometry, thus: From the square of the radius subtract the square of half the given chord, and the square root of the remainder will be the distance of the centre of the circle from the

were borrowed for 30 years, at 6 per cent., what annual sum would pay it middle of the chord; subtract this distance from the radius, and the remainder off in that time ! Ans., £100. For £7905.8183 is the sum borrowed; will be the sagitta (or distance from the middle of the chord to the middle of

100+6

7905.81838.06 at

=1.06; and a, = a1 the arc); add the square of this sagitta to the square of half the given

= 5.743191; therefore u = 100

5.743491 - 1 chord, and the square root of the sum will be the required chord of half the arc. Otherwise : subtract the square of half the chord from the

= 100: whence the answer is, £100 must be paid yearly. square of the radius, and extract the square root of the remainder. Subtract this root from the radius, and multiply the remainder by twice the radios; then extract the square root of the product, and it will be the required chord of half the arc. The versed line is the difference between the radius and the cosine. See No. 71, part 18.-J. F. TREGS (Glasgow):

LITERARY NOTICES. Isolated is pronounced sce-o-lat-ed.

GRAND DOUBLE NUMBER OF T. K. (Clerkenwell): We wish his answer to the Four Ball question had been right; we have had many solutions, but it is not yet answered.

THE ILLUSTRATED FAMILY PAPER." UN ETUDIANT (Darlington): Most likely a Greek Dictionary, &c., will be published.-T. F. (Holborn): The difficulty which he meets with in regard “Cassell's Illustrated Family Paper,” No, 13, bearing date March 25, will to exercises in parsing, or any other exercises, would be met by his joining be a Double Number, containing 16 Pages, price Twopence. This a mutual instruction society, a thing we bave often recommended. If any

Double Number will be magnificently Illustrated, and will be sure to six young men, or old men, or men of mixed ages, agree to meet and assist each other, they can supply the place of a tutor among themselves : and if

command the highest approval of all Purchasers. any serious difficulty occurs, we are willing to become umpire. It is quite

TAE WORKING MAN'S FRIEND AND FAMILY INSTRUCTOR. possible to become a correct Latin scholar, or any other scholar, by doggedly studying the Lessons in the P. E.; but the mutual instruction system

Re-issue of this celebrated and popular Work. would greatly enlighten and shorten the process. It would even greatly This Work is offered, for a limited period, at a reduced price. The set add to the power of the system, as well as to its interest and its humanity, comprises seren volumes, crown octavo. For the excellency and general if the six mutual instructionists would take some ragged and destitute interest of its contents it is not be surpassed, it forms a library in itself, juvenile delinquent from the streets, and try to instruct him and make him including Tales and Narratives 'by William and Mary Howitt, Miss Metc. a better member of society. By endeavouring to Instruct such a one, they yard (Silverpen), and other writers of eminence ; leading articles on tópics would learn much better themselves, and they would see the human mind of deep interest to the working classes; the celebrated letters of Martha expanding under culture, like a drooping plant, transplanted into a new Makepeace on Domestic Economy; Papers and Extracts on Science and and better soi!. Oh the pleasure of doing good! They would be amply Art; Parson Frank's Short Homilies ; Original Biographies; Poetry, &c., rewarded. Their mercy would be twice blessed; it would bless both them &c.-Also, in two volumes, the same fornu and size, that gave and them that received the precious boon. Remember “ that the soul be without knowledge is not good."

“THE LITERATURE OF WORKING MEN," ECOLIER (Wisbech): We shall take up the subject of Architecture consisting of Prize Articles, Tales, Essays, and Poetry, written exclusively again, when we can get an opening in the P. E.-J. CUILD (Tenby): Logarithms when we can get room in the P. E.- INQUIRER (Bideford): We don't series without inconvenience, the volumes will be issued Monthly, 18. each,

by Working Men. In order to enable individuals to obtain this valuable think much of the books to which he refers; as to the Elementary Bodies

in cloth boards. The First Volume will be ready with the Magazines for in Chemistry, we hold in our hands a work of some value, which

gives a list April next. It contains 360 closely-printed pages, including in its contents of 64 with their names, equivalents, &c. These, of course, will be given in order, in the Lessons in the P. E.-A. B. (Carnarvon): We do not know any Street Boy," by Miss Meteyard (Silverpen); the History of John Weldon;

“ The Wood-nook Wells,” a powerful Tale, by Mrs. Mary Howitt; “The Pye College where students are admitted gratis; but what connexion this has

Cromwell and his Times; Letters on Household Economy, by Martha with being a British Consul or Ambassador, we do not see 1-Tom (Dublin): See our concluding Lesson on Music.-OPIPBX (Eaton Square) will find a Essays, addressed to the Working Classes; and a large amount of enter

Makepeace; Biographies of Eminent Statesmen and Authors; Original description and drawing of the Marquois Scales, or parallel ruler, in the tainment and instruction in Prose and Verse. The volumes will only be P. E., vol. 1., p. 49, col. i, fig. 5, and line 13 from the bottom:-CoQuus issued on the above-nained terms to persons subscribing to the whole set. (London): There is no such thing as perpetual motion inventible by man. M. GUTÁRIB (Liverpool, Box W. 90) wishes an accession to a society of

CASSELL'S LATIN DICTIONARY, BY J. R. BBARD, LL.D.-ihe public young men who are engaged in carrying on a manuscript magazine. As

are respectfully informed that the publication of this Dictionary has comyoung men best understand one another, we hope this intimation will menced, and that it will be issued regularly every week until its completion, incite some unfledged geniuses to come forward and aid the good cause

which will be in about Twenty-six Numbers, TBREEPENOB each, or in by their contributions. We hope, however, that these contributions will Monthly Parts, ONE SHILLING each. Part the First is now reads; Part not be like those which the Professor, who used to meet his students in his the second will be ready with the Magazines for March. own chambers finely carpeted, instead of tho class-room, requested them

CASSBLL'S FRENCH AND ENGLISH DICTIONARY.-The FRENCH and to leave at the door.–1. GRIFFITHS (Aberystwith): 8ce pp. 137 and 213, ENGLISH portion of this important Dictionary is now completed, and may be vol. 11., P. E. ; also pp. 119, 295, and 323, vol. III.; and pp. 207 et seq. had, prico 48., or strongly bound, 5s. The ENGLISH and French portion vol. IV.

is in the course of publication, and will be completed in about Twelve U. J. CHRYISTB (Burnley) : Right.-H0 HYPOMERON, D. JOSEPH (Sun- Numbers, THREEPERCE each. The entire Dictionary, forming one handsome derland), ana Wm. Keys, (Hythe): Three or four numbers will complete volume, will be ready with the Magazines for April, price 98.6d. “Cassell's French Dictionary; and it is expected to be finished in five or Cassell's GERMAN PRONOUNCING DICTIONARY.-The GERMAN six weeks.-C. L. D.(Wales) should study at any one of the affiliated colleges English Portion of this Dictionary is now ready, price 58. in stiff covers, of the U. of London he pleases; also matriculate, and take degrees, but or 59. 60. strong cloth,-The ENGLISH-GERMAN Portion will be completed say nothing about his domestic concerns.-D. H. (Driffield): His im- as quickly as possible, in Numbers, THRERPENOB each; and the entiro portant communication is under consideration.-GRATUS (Bodmin): Right, 'Volume, strongly bound, at 9s., will shortly be issued.

called the ascension-pipe. At a second stroke of the piston, the ON PHYSICK OR NATURAL PHILOSOPHY. same series of phenomena are reproduced ; and after a few No. XXIII.

strokes, the water, following the air, finds its way into the

barrel. The effect of the succeeding strokes is now modified, PNEUMATIC AND HYDRAULIC MACHINES. During the descent of the piston, the valve is closed, the com(Continued from page 320.)

pressed water raises the valve o, rises above the piston, which

then lifts it up and causes it to rise by degrees to the upper Pumps.—Pumps are machines employed to raise water, by reservoir. There is, after this, no more air in the barrel, and guction, by pressure, or by the united effect of both : hence, the water, urged by the atmospheric pressure, rises with the their division into three kinds, the suction or common lifting piston, provided that the top of the barrel is no higher than 34 pump; the forcing pump; and the suction and forcing pump, or feet above the level of the water in the lower reservoir, which lift and force-pump. Before the time of Galileo, who died in receives the lower end of the suction-pipe ; for we have seen, 1642, the ascent of water in suction-pumps was attributed to in a former lesson, that a column of water of about 34 feet Nature's abhorrence of a vacuum ; but we now know that the balances the pressure of the atmosphere. cause is the pressure of the atmosphere.

In order to ascertain the proper height which may be given The Suction- Pump.-The suction or common lifting pump to the suction-pipe a, it should be observed that in practice has been already so far explained in a former lesson; but in the piston is never brought down exactly to the bottom of the order to make the subject clearer to some readers, we shall barrel, and that there is always below it a small quantity of explain a model adapted for lecture-room demonstration, as air having the force of the atmospheric pressure. Suppose the shown in fig. 112. This model is composed, 1st, of a cylin. space which this quantity occupies to be sto of the barrel.

Then the air in this space expands as the piston ascends, and Fig. 112.

when it is at the top of the barrel, the tension of the air in the barrel is so of that of the atmospheric air, according to Mari. otte's law. The air of the suction-pipe, therefore, cannot be rarefied beyond this limit; and consequently the water, in this case, can only be raised to the height of 3 of 34 feet, that is, to the height of about 33 feet. This height is still too great, because the water must be raised by a certain quantity above the valve s; so that, in general, we cannot give to the suctionpipe a greater height between the limits above mentioned, than that of about 26 or 27 feet. Thus we see that in the suction-pump the water is first raised in the suction-pipe by the force of atmospheric pressure, and that the height 'thus obtained does not in general exceed 27 feet; but when once the water has passed through the piston, it is the ascensional force of the piston which raises it, and the height to which it may then ascend depends only on the force which moves the piston.

The Forcing-Pump.—Having explained the nature of the common pump, erroneously called suction-pump—seeing that the suction of the water, or its ascent in following the piston, is due simply to the pressure of the atmosphere, and that it would fail in so doing as soon as the column of water exceeded that pressure, the limit being at the utmost within 34 feet-we proceed to describe the operation of the forcing-pump, repre, sented in fig. 113: a is the suction-tube, having its lower end

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'Fig. 113.

D

drical barrel made of strong glass, at the bottom of which is a valve 8 opening upwards ; 2nd, of a suction-pipe A, which is immersed in the reservoir containing the water to be lifted; 3rd, of a piston with piston-rod which rises and falls in the barrel, the piston being perforated in the centre, and having the orifice covered at top by a valve o opening upwards. The pump-handle p is employed to put the piston-rod and piston in motion. When the piston is raised from the bottom of the barrel, a partial vacuum is formed below it, and the valve o remains closed in consequence of the pressure of the atmo- immersed in the water as before. pis a solid piston without a sphere ; while the air in the suction-pipe a rises by its elastic valve, which moves in the body of the pumpe, by means of a force, raises the valves, and partly forces its way into the barrel. lever as in the common pump. The air is withdrawn from The air below the valve s, being thus rarefied, the water rises the suction-tube as before; but instead of escaping through a in the suction-pipe until the pressure of the liquid column so valve in the piston, as it cannot return through the valve r, it raised, added to the tension of the rarefed air remaining in the is forced by the descent of the piston p up through the valve l, pipe, balances the external atmospheric pressure on the water into the ascension-tube •; the water inen follows by the presin the reservoir.

sure of the atmosphere and the ascent of the piston, and is When the piston is lowered, the valve s is closed by its own forced through the valve r; this valve is then closed by the weight, and prevents the return of the air from the barrel into descent of the piston, and the water in the body of the pump the suction-pipe. The air in the barrel, being now condensed is forced through the

valve l, and up the ascension-tube , by the pressure of the piston, opens the valve o, and escapes om which it cannot return, as its weight shuts the ralve l. into the atmosphere by the pipe above the barrel, which is This process is cninued until the water in the ascension.tube VOL. IV.

101

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za raised to the required height, the force necessary to raise it received, reacts on the water and raises it in the tube D, until by pressure on the piston continually increasing until it the piston re-descends; and in this manner, the jet continues reaches that height, and is discharged from the ascension-tube. without intermittance.

In another form of the forcing-pump, there is no use made The Load of the Piston.-In the pump just described, after of the pressure of the atmosphere. The suction-tube, of course, the water has filled the suction-pipe and the barrel up to the is dispensed with, and the pump-barrel is itself immersed in mouth of the jet, the force necessary to raise the piston is equal the reservoir of water to be raised. The continuity of the jet to the weight of a column of water having for its base the

, a vessel or reservoir of air, which we shall explain in the follow- distance of the orifice of the jet from the level of the water in ing description of the pump called the lift and force pump, and the reservoir from which it is drawn. Thus let II be the pressoinetimes the section and forcing pump; which differs but little sure of the atmosphere, A the height of the water above the from that just described, except in the application of the air- piston, and h' the height of the column of water which fills the vessel to produce continuity in the flow. This object is also suction-pipe and the lower part of the barrel. The pressure accomplished by the operation of two pumps acting alternately, above the piston is evidently uth, and that below the piston as in the construction of the common fire-engine.

H-h', since the weight of the column l' tends to balance the The Lift and Force Pump. -- This pump raises water both by pressure of the atmosphere. Now, the pressure --h tends to Buction and by pressure, that is, both by atmospheric pressuré raise the piston; the effective resistance, therefore, is the excess and by mechanical pressure. It has a solid piston, and at the of i+h, above -h'; that is, h-rk', which was to be proved. bottom of the barrel on the top of the suction-pije, a valve

e

Practical Application of Pumps.-In practice, the following 8 opening upwards, fig. 114. Another valve o, also opening rules are observed in the construction of pumps. The velocity

of the piston is calculated to vary from six to nine inches per Fig. 114.

second. The area of the aperture covered by the valves is about half that of the barrel of the pump. The diameter of the Buction-pipe, and of the ascension or discharge-pipe, is about two-thirds of that of the barres of the pump. The stroke of the piston in large pumps varies from three and a half to five feet. In good pumps the loss occasioned by the time required for shutting the valves reduces the effect to about four-fifths of

that produced by the piston. The following figures present D

different models of pistons and valves employed in the construction of pumps. Fig. 115, a piston packed with leather ; fig. 116, a piston packed with hemp; fig. 117, a piston furnished with a single clack-va ve; fig. 118, a piston with a double-clack or butterfly valve ; fig. 119, interior of the barrel

of a pump, in the bottom of which a single clack-valve works ; B

fig. 120, separate view of a single clack-valve ; fig. 121, a

conical valve.
S

Fig. 115.
Fig. 116.

Fig. 117.

wpwards, closes the aperture of a bent pipe, which, passing under the metal plate a, connects the valve s with the air-vessel M. From this vessel or reservoir of air, proceeds the ascension

Fig. 118. pipe D, which is employed to raise the water to any given

Fig. 119.

Fig. 120. height. At every ascent of the piston B, the water rises in the suction-pipe A and enters the barrel. When the piston descends again, the valve s is shut, and the compressed water raises the valve o in order to enter the reservoir m, and thence pass into the ascension-pipe , in which the height attained by the water is only limited by the force of the moving power which keeps the pump in operation. If the pipe D were only the continuation of the pipe J, the flow or jet of water would be intermittent, taking place only when

the piston descended, and stopping as soon as it ascended. But the continuity of these tubes is interrupted by the air-vessel m, by means of which a continued jet is maintained. The water thrown into this vessel is divided into two parts, of which the one raised in the tube D compresses the water in the reservoir m; while the other, in consequence of this pressure, is raised in the reservoir above the lower orifice of the pipe D, by the compression of the air which is above it. Consequently, when the piston ra-asconds and no onger acts so as to compress the water, the Bramak's Hydrostatic Press.-We have already given an air of the reservoir by the excess of pressure which it has | explanation of this invention : but as it is connected with the

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explanation of the forcing-pump, we repeat it here under succession, and let us suppose that the lower extremity of another form, its importance demanding additional elucidation. the tube g is placed between the two ajutages b and c. The hydrostatic press is a beautiful application of a principle 1st. If we open first the ajutage b, the water runs out, its previously known for nearly two centuries, and commonly level is lowered in the tube g, and as soon as this lovel reaches called the hydrostatic paradox; viz., that any quantity of water, that of b, the run of water is stopped. This phenomenon is however small, may be made to balance any quantity how- explained by the fact of the excess of inward pressure which ever great. The practical effect of this principle is, that when took place first at b; an excess which disappears when the water enclosed in a vessel quite full of the liquid, is pressed level of the water in the tube g is brought down to b. For, by a piston at any aperture with a given force, this pressure is before the water issued from b, the pressure on all points of the ai once communicated to every part of the vessel of the same horizontal stratum be was not the same. Ate, it was composed size as the aperture, with the same force. Mr. Bramah, by an of the pressure of the atmosphere and the weight of the column ingenious application of the forcing-pump to an apparatus con- of water ge, whilst at b, the pressure was that of the atmosphere structed on this principle, produced one of the most powerful only. But as soon as the level of the water is the same at o and useful machines used in the present day. It is represented and at 6, there is an equilibrium, because that in the bottle in fig. 122, where . is the piston which moves in the cylindrical and in the tube the pressure is then the same on all points if

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tube f; or small barrel of the pamp; p is the piston which the horizontal stratum be. Thus, the pressure at & and sto moves in the cylinder od, or large barrel of the pump; and being equal to that of the atmosphere, it is easy to demonstrate at bu is the tubé of communication between the two barrels of that it is the same pressure which acts at any point o of the the pump. A lever of the second kind I raises the pistons, stratum be. For this purpose, let 1 denote the pressure of and the water in the reservoir 6 is drawn into the barrel of the the atmosphere ; this force acting at o atid e, is transmitted in pumpf. When the lever is preesed downward, a valve shuts, and prevents the water from returning into the reservoir Ó',

Mg. 193. and forces it along the tube tbt, in order to act upon the lower extremity of the piston p, to which is attached the plate p'; ef is another plate, against which the objects to be compressed by the machine are pushed by the former. In consequence of the quaquaversal pressure of the water forced into the large barrel of the pump from the small one, the pressure of one pound on every square inch of the surface of the liquid in the latter will be communicated to every square inch of the surface of the liquid in the former. Hence, if the diameter of the piston s be one inch, and that of the piston 6 be ten inches, the pressure of one pound on the former will be 100 lbs. on the latter. A noble specimen of this press was exhibited in the Crystal Palace by, the Bank Quay Foundry Company, Warrington; viz., that which was used for raising the Britannia Tubular Bridge. The greatest weight lifted by this press was 1,144 tons, and the quantity of water used for every lift of six feet, was 81} gallons. The internal diameter of the great cylinder was twenty-two inches, and that of the ram or piston twenty inches.

all directions in the interior of the bottle, according to the Mariotte's Bottle. - The bottle of Mariotte is an apparatus principle of Pascal formerly explained, and the upper side k which exhibits some remarkable examples of the pressure of resists an upward pressure equal to H-ko; for the weight of the atmosphere, and by means of which we may obtain a con- the column of water ko pattially counteracts the pressure stant flow. The neck of this bottle is closed by a cork, through which is transmftted to k. Now, according to the mechanical which a glass tube passes, open at both ends ; in the side of principle, that reaction is always equal and contrary to action, the the bottle there are three apertures furnished with ajutages, pressure -ko is urged downward by the side k on the stratum 1, b, c, at right angles to the side and closed by wooden pegs.be; so that the particle of water at o, supports in reality two When the bottle and the tube are quite full of water, let us pressures, the one equal to the weight of the column of water vonsider the effect of opening one of the ajutages, a, b, c, in ko; the other, the pressure #—ko, resulsiog from the reaction

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of the side k. The real pressure, therefore, which the particle remembered that inasmuch as our desire is not merely to at o supports is k 04-1~k o, that is, h the pressure of the atmo- discover a certain metal, but a given compound of a certain sphere ; which it was proposed to demonstrate.

metal, we shall require not merely a test for mercury, but for 2nd. If we shut the ajutage b, and open the ajutage a, there that which is combined with the mercury and holds it in soluwill be no run of water; on the contrary, the air will enter the tion. Let us begin with a very characteristic test for soluble bottle by the aperture a, and the water will rise in the tube g persalts of mercury generally—a solution of iodide of potasto the level ad, when the equilibrium will be restored. For itsium. The effects of this test are most remarkable, as will be is easy to perceive by reasoning, as in the preceding case, that seen. Having poured a weak solution of bichloride of merthe pressure is then the same at all points of the horizontal cury into a test-tube or a conical glass, add to it, drop by drop, stratum a d.

another solution of iodide of potassium, and remark the beau3rd. Let the apertures a and 6 be closed, and the aperture c tiful play of colours which result, also the disappearance of all be open. In this case, there will be a run of water with a con- colour, all precipitate, and the resumption of perfect transstant velocity so long as the level of the water in the bottle parency in certain states of dilution; that is to say, so soon as does not fall below the level of the lower orifice l of the tube : à certain amount of test liquor has been added. It is useless for then the air would enter this orifice and fill the upper part to expatiate on changes which can be seen very much better of the bottle, where it would take the place of the water run than they can be described, but I may remark that by revers. off. In order to demonstrate that the flow of water at c is con- ing the order of experiment in testing, and adding the bichlo. stant, it is necessary to show that the pressure on the horizon- ride solution to the iodide of potassium solution, instead of tal stratum ch is always equal to the pressure of the atmo. adding the latter to the former, the chromatic effects will vary. phere increased by that of the column hi. Suppose, then, that Now the peculiar effects here detailed are characteristic of a in the bottle the level of the water is lowered to the stratum persalt of mercury; and the only persalt of mercury at all ad. The air which has entered the bottle supports a pressure likely to come under one's notice in a case of poisoning is the equal to 1-on. In consequence of its elasticity, the air perchloride of mercury; however, any doubt can be at once transmits this pressure to the stratum ch. Now, this stratum cleared up by the addition of the test for chlorine. Now what supports besides this the weight of the column of water p m. is the test for chlorine ? You remember, I presume, that it is Therefore, the pressure transmitted to m is in reality, pm +unitrate of silver-the reaction of that substance having already, -pn, or 1+mn, that is, 1+hi. In the same manner, it under the head of Silver,” come before us. Add, then, a little would be demonstrated that this pressure is the same, when nitrate of silver solution, and a white precipitate results. But the level is lowered to eb; and so on, so long as the level is there are thousands of white precipitates ; how shall we know above the orifice 2; the pressure on the stratum ch is therefore what this one is ? Simply by adding hartshorn (liquor ammoconstant, and so is the velocity of the flow. But as soon as the niæ) to it, the white precipitate dissolves. It has been prolevel falls below the point l, this pressure decreases, and con- duced, therefore, by chlorine. We have already determined sequently the velocity of the flow diminishes. Thus, we see the presence of mercury, therefore our substance must be a that the bottle of Mariotte is a means of obtaining a constant chloride of mercury. But as there are two chlorides of mercury, flow; namely, by filling it with water and opening the ajutage which is this? It cannot be the proto-chloride of mercury or aperture placed below the orifice 1 of the tube.

The because that substance is insoluble; it must, therefore, be the velocity is then proportional to the square root of the height per- or bichloride of mercury. Thus, at length, we arrive at ih, as shown in a former lesson.

the demonstration.

The next test we shall employ is ether; a liquid which, by the way, is rather to be considered as a separator than a test in the ordinary acceptation of the term.

Having poured a little bichloride solution into a narrow LESSONS IN CHEMISTRY.-No. XXII.

test-tube, add to it about an equal volume of rectified ether In the course of any chemical examination, and more espe- always mean sulphuric ether), then closing the tube with

(when the term ether,” without prefix, is used, chemists cially of metals, we find certain compounds demanding our the thumb, agitate the tube. Proceeding thus, the ether and especial attention. Amongst all the combinations of mercury: the bichloride solution will become intimately mixed. This perhaps the bichloride, or corrosive sublimate, is that which misture being effected, cork the tube (to prevent evaporation covery and identification, when mixed with animal fluids, minutes at rest. Presently two distinct Auid layers will be involve many points of chemical interest, and the manipula- recognisable ; 80 well marked, so thoroughly individualised, tive processes employed in its extraction are of great beauty that one may be readily separated from the other. The best and delicacy.

I must premise by directing the student's attention to the means of effecting the separation is, as represented below, fig. fact of there being two chlorides of mercury-one the proto-8, by the use of a little glass instrument termed a pipette. chloride, ordinarily known as calomel; the other, bichloride, ordinarily known as corrosive sublimate : the respective com

Fig. 8. positions of which are as follow :

Parts by weight.

Chlorine. Mercury.
Calomel, or Proto-chloride of
Mercury :

36

200 Corrosive Sublimate, or Bi. chloride of Mercury

72

200 Hence the ratio of chlorine in these two chlorides is as one to two. Calomel may be generated in various ways. The student has already generated it by the addition of common-salt solution to proto-nitrate of mercury, and he will not fail to see, by reference to the preceding tabular exposition, that if by any process we can succeed in taking from a given weight of sublimate half the chlorine it contains, the result will be calomel; and if we can succeed in removing the whole of its chlurine, the result will be metallic mercury.

Chemical investigation of Bichloride of Mercury in simple and omplex fluids. Taking a portion of the solution of bichloride f mercury already prepared, let us master the appearances it The student, however, who does not possess the instrument ffords with tests in simple solution. And here it will be may accomplish his result perfectly well by means of a glass

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