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Cr be a perpendicular drawn from the vertex c to the plane! This question was also solved by Quinun Pringle Glas-
ABD at E, and join EA, BR, and ed.

gow --and we shall insert his solution, probably, next week,
in it is different trom the preceding one--and by one or two
others.
1

SOLUTION OF THE CAST QCERY.

2. 332, 11. IV.)
Fill the three-hailin cask out of the eakt-gaiion cask, and
pour its contents into the fire-gaiion cask ; till the three-Jalijs
cask gain out of the right-salim cask, and till up the five-hailon
cask from it, you will then have one gailon of water left in

the 'hree-galian cask and two gallons of water in the evht-yailon
DLOO

cask. Now, empty the fire-justun into the enghe-faiion cask,
I and you will have seren zations of water in it; pour the one
gation of water out of the three-daliun cask into the five--alion
(ask, retil the three-hallon cask vut of the curghtniion cask, and
pour its contents into the fire gallos cask; you wiil then hare

tour gallons of water in the fre-allon cask, and four zulons Then the print e, and every other point in ce, is equally, of water in the eigint-valion cuek; wiich was to be done. diqtant from the points A, B and e.

This question was solved by J. Beckett, Shrewsbury; J. W.

Porce being the common 3., Spilsoy; C. Thomas, Si. Austell: E. Lawford, Laton; perpendicular to each of the straight lines i a, s it, and 2 German, Ling Acre ; and 3 considerable number of other (Euclid xi. Def. 3), and the edges a C, BC and Dc, heing equal, the bARe BA, ?? and ? n, are equal Euclid 1, 47). Also, it

correspondents, any point be taken in ce, its distance from the point o will be the common perpendicular to three triangles, whose bases are PA, and D; and these being equal, the distances of that

ANSWERS TO CORRESPONDENTS. point from A, B and », ire also equal Euclid s. 17).

W. Rinformed, on the best anthority, that age is no hindrance to Again, if pebe produced to meet a B, it will bisect it at Matriculation in the Civersity of London after 1 certain period, that is right angles in P, For, the angle DAR is equal to the angle. after the ace of 16 years, arv one nay matriculate at whatever age ne DBA, and the angle EAB to the angle rss Euclid 1. 5); pleases. As 'o the attendance in person at the apartments of the University therefore, the remaining angles DAB and D B 2 are equal, hence in romerset flouse, the week before the Examination-week will answer te

As to the gubjects, the Candidate must know what is required the angle. De and an are also equal, since they are respec- in Chemistry, Classics, English, French, etc., according to the Regulations tively oqual in the anglea Dagand she (Euclid 1,5). There. I inserted in the P, E. vol. 1. p. 137. fore the triangles top and IDF, are equal in every respect sons, and ihat Dr. Beard ufrformed ali that he promised, and mure thao we

JAMES REAVE: We think his friend is mistaken as to the Laua Les (Puclid 1, f;; whence A e is equal to RP, and the angle A P D 41dbut we 211.cerely thank ain for his friendly binte.-W. H. TYSON to the angle EPD Honce, AB is bigected at right angles Hulme : Vany thanks for sig list of errata.--. Tæompson Pool; should by DP (Puclid 1. Def. 101,

solve oor Rall question before he proposes his own.-W. GRIMXOND atji

Thanks for Dis notes on the Algebraic questions. Now, let another perpendicular Do, he drawn from the

UR N. Manchester: The Balances in the Ledger ought to be entered angular point n to the plane ABC, and it may in like manner a new in the Journal as 188Pts and itaðnútres, in order to preserve the be shown, that every joint in this perpendicular is equally distant accuracy of the system of creck, which depends on the total additions of frmn the points A, H and , and that Go, produced to meit. A B,'

each side of the Ledger being the saine astuse si the Jurnal for any

giren period. biscote il mt right angled in p. But the straight lines joining D, C GLENTINE (Johnstone. Under consideration.-W.1. B.: The specific and r, are in the same plane Euclid xr. 2, i hence the straight' gravity of water being : 000, the specific gravities of other bodies are usually line* Ce and Fri), which hare their extremities in this plane, venin accordance with this unit, 9:2. one .housand; this inethod prevents

the necessity of 118ing fractional paris, unies, one wishes to be very precise. are also in the same plane (Euclid. XI. 1), therefore these

With regard in water, the weight of a cubie ot of this iqlių is commonly perpendiculare intersect each other; and since every point in ce reckoned at 1000 oances, which is only approsimate y true; for by the ig equidistant from the points A, B and », and every point in experiments on which the regulations on this point in the act of Parliament

for ealabiishing " Uniformity in Weights and Measures" were founded, it DO19 fquidistant from the points A, B and C, their common

appears that" i cubic inch of distilled water weighed iu air by brass welches point of intersection o, is equugisiant from all the four angular, at the temperature of 62" Pahrenheit's thermometer, is equal to 250 15* pointa,

grains, Troy weight;" and that the gaiion shouid contain lu pounds of The walculation of the distance Do is easily effected in the distilled water weighed in air at 6.- Fahrenheit.” Now, by proportion,

We have 4'45 grains : 10 lbs. or TV,000 grainsii I cubic inch: 1787384 following manner:

cubic inches, or, as it is generily stated, 17-274 cubic inches, taking the Since the straight linear and se produced will bisect the nearest thonsandth part of an inch. But at the temperature of 39. Fahrer opposite sides D p and AD, as ne has been shown to do in the best that of the maximun density of water, the Ryal Commissioners case of AA, we have the well-known property that 1 Eis?DP; ther-fore, we have the following proporcions, i énbic inch : 1708 puble inches and that , which is equal to DF, is similarly divided in o, :: 2.23 grame : 137,184 grains; and I lb. or 1000 grains : 137.164 graias; and op je equal to Op. Also, from the similar triangles Dar 18 23. : 999-2777 023., which is very nearly 1000 des., wanting only about it

? and Do we have

of an ounce, or lig drams. By the same mode of caiculation, the weight

of * cubic fool of water would be found to be 997 1369 028., which wants Dop: 1p :: DB2 : 2 (Euc. vi. 22.)

about 2 ounces of the 1000 ounces ; so that every thing depends on the but, Do? =D ? Or? (Euc. 1. 17.)

temperature of the water in accurate calculations. , ? = 1) F? – SD F)? 'hy substitution),

A WOOD Manchester): There are no works og Pneumatics and Hydrau= D. $0 po? (boy aquaring),

lics more inodern than the Lessons in the P. E.-J. R. (Huin): Would any

reader inform him of a work or tue Rudiments of Water-culour Draving ?-=&p? (ny subtraction);

J. WRAGG (Retford, proposes to form with others a society or class to alen, D p = $D$? (hy squaring).

studs Botany, and requests our advice as to which system should be adopted;

of course, we recommend the natural system. Hence the above proportion becomes

E. 6. SMITH (Lambeth) : We do not see how we can forward bis views.

MARY ANNE Reading, writes us a lively and amusing letter, requesting ? ; D) $3 ;. $D$? : D (by substitution),

our opinion of certain English Grammars she has studied, and aske us Do you: D (by alternation and dividing whether she caght to know enough to write and speak correcus. We think the two first terms by $ DP4); the lessons in English

in the P. E. are superior to any of those she has meo

tioned. In her letter, we have counted no less than tinteen errors in spelling

100 - 23 whence, do

= 37:5, alone! She ought to know better. We strongly recommend to her perusal, 2

2

Watts' "Improvement of the Mind," but not Gulliver's Travels. We

cannot tell about the recipe for the Palladium or tooth-metal, and hope she Orf, DO = V 37 6 = 6:12372435 inches.

does not require it for her own use.
Therefore, taking the radius of one of the four balls from a gently disapprove. Etiquelle is pronounced šy-ceě-kéit ; and again,
thi# Vai nf po, we have the radius of the interior ball
goto 1 12472435; or it# diameter 2.217487.

Vol. iv. p. 329, col. 2, line 32 from top, for the whole, read half the whole.
X PLUS Y.
» D. 328, col. 2, line 19

for 13 74, read 25 21.

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the dial; but in proportion as the rod a gets heated more and : ON PHYSICS, OR NATURAL PHILOSOPHY. more, the index is seen rising by degrees, and thus the elon.

gation of the rod is rendered sensible. No. XXIX.

The cubic expansion of solids is proved by means of the (Continued from page 19.)

ring of S'Gravezande. This name is given to a small metallic

ring through which, at the ordinary temperature, a copper CALORIC.

sphere, having nearly the same diameter, readily passes. But NATURE OF HEAT; THERMOMETERS.

when this sphere has been heated by the flame of a spirit-lamp,

it can no longer pass through the ring, turn it which way you Hypothesis on the Nature of Heat.—The name of caloric is given will—a proof of its increase in volume, or of its cubic expansion, to the agent which produces within us the sensation of heat. that is, expansion in all directions. This agent acts upon all bodies animate and inanimate; it In order to demonstrate the expansion of liquids by heat, a melts ice, boils water, and makes iron red-hot. Numerous small hollow glass ball is united to a capillary tube, and the opinions have been formed as to the cause of heat; two only ball with part of the tube being filled with any given liquid, remain in the present state of science: the system of emission, we observe that, as soon as it is heated, the liquid rises in the and that of undulations. In the former, it is supposed that the tube, and the expansion thus observed is always much greater cause of heat is a material and imponderable fuid which passes than that of solids. The same apparatus may be employed to from one body to another, and whose particles are in a state of prove the expansion of gases by heat. For this purpose, the continual repulsion ; and that this fluid exists in all bodies, in ball is filled with air or any other gas, and into the tube a a state of combination with their ultimate particles, and mercurial index of about an inch in length is introduced. opposed to their immediate contact. In the system of undu. When the ball is heated, by merely bringing it into contact lations, it is supposed that the cause of heat is a vibratory with the band, the index is driven towards the open extremity motion of the particles of heated bodies, which is trans of the tube ; and, if heat be applied long enough to the mermitted to the particles of other bodies through the medium cury, it will ultimately be expelled from the tube. Whence we of a fluid wonderfully subtle and elastic, which is called ether, conclude that, even with a slight increase of heat, gases are very and in which it is propagated in the manner cf sonorous waves expansible. In such experiments as these, as soon as the bodies in the air. The hottest bodies are, consequently, those whose

are cooled, they contract and assume their original volume, vibrations have the greatest amplitude and the greatest velocity, when the heat is reduced to the same degree as before. and heat is nothing else but the resultant of the vibrations of

MEASURE OF TEMPERATURES. the particles. On the former hypothesis, the particles of bodies which are cooled lose their caloric; on the latter, they Temperature and Thermometers.—The temperature of a body only lose their motion,

is the actual state of the sensible caloric in the body, without The progress of modern physics appears to be entirely in increase or diminution. If the quantity of sensible heat favour of the theory of undulations. Yet, as the theory of increases or diminishes, we say that the temperature is raised emission simplifies the demonstrations, it is generally preferred or lowered. The instruments which are employed to measure in the explanation of the phenomena of heat.

temperatures and ascertain their variations are called thermoGeneral Effects of Caloric.—The general action of caloric on meters, that is (from the Greek), heat-measures. The great bodies is the development among their particles of a repulsive difference and variety of the sensations of heat excited in force incessantly struggling with molecular attraction ; it different individuals by the same amount of caloric in a body, therefore follows that, under the influence of this agent, bodies prevent us from measuring this amount with any degree of iend at first to expand or dilate, that is to assume a larger certainty by means of these sensations; we are therefore volume; then, to change their state, that is, to pass from the obliged to have recourse to the physical effects of caloric upon solid to the liquid state, or from the liquid to the gaseous certain bodies as a method of determining the degrees of heat state.

in all others. These effects are of various kinds; and those of All bodies are expanded by the action of caloric. The most expansions and contractions have been generally adopted, as expansible bodies are the gases ; the next are the liquids; and being the most easily observed and the most readily recorded. lasily, the solids. In the case of solids, two kinds of expansion But heat also gives rise in bodies to electrical phenomena, by are considered : 1st, their linear expansion, that is, in the direc- means of which temperatures can be measured, and extremely tion of one dimension only, say the length; 2nd, their cubic sensible thermometers have been constructed on this principle. espansion, that in the volume or bulk of the solid. These Of all bodies, in general, liquids are the best adapted for the expansions, nevertheless, take place at the same time, under construction of thermometers, solids not being sufficiently the action of heat, In liquids and gases, it ts only the expan- expansible, and gases being too much so. The liquids exelusion in volume that is taken into consideration.

sively adopted are mercury and alcohol ; the first, because it In order to determine the linear expansion of the metals, the enters into a state of ebullition at a very high temperature ; apparatus represented in fig 156 is employed.

and the second, because it has never been solidified, that is,

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A metallic rod A is kept fixed at one of its extremities by a frozen, by the greatest known degree of cold. The thermotightening screw B, while at the other extremity it is free and meter was invented about the end of the sixteenth century, in contact with the shorter arm of an index

k, moveable on a Its invention is attributed by some to Galileo ; and by others, dial . Below the

rod is a cylindric reservoir containing burn- to Drebbel, a Dutch philosopher, or to Sanctorius, a Venetian ing alcohol or spirits of wine At first the index is at zero on physician.' The mercurial thermometer is that

most exten107

VOL. V.

sively used. It is constructed of a capillary glass tube, to one found that the column of mercury varies considerably in its end of which is blown a cylindric or spherical reservoir, fig. length, the tube is thrown aside, and a more regular one 157.

substituted in its place. But if the variatims are small, a slip

of paper is pasted on the tube lengthwise, and a mark is made Fig. 157,

with a pencil at the points successively orrupied by the extremities of the column of mercury. The divisions thus obtained necessarily indicate equal capacities, since they correspond to the same volume of mercury. Now, the intervals of these divisions being sufficiently uniform to admit of their being considered of equal diameter throughout each of them, the smaller divisions are obtained by dividing each of the former into a certain number of equal parts; and this, as was observed in our first lesson, may be accomplished by means of the dividing machine. By these divisions, an exact graduation of the scale is effected.

Introduction of Mercury into the Tube.--In order to fill the bulb of the therinometer with mercury, there is blown to the upper extremity of the tube, a funnel c, fig. 158; which is first filled with mercury; then, by inclining the tube a little to

Fig. 138.

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This reservoir and part of the tube are filled with mercury, and the expansion of this liquid is indicated by a scale either graduated on the tube itself, or on a metal rule placed parallel to it on a case or fraine. Besides the operation of joining the tube and the bulb, the construction of the thermometer includes three other important operations : 1st, the division of the tube into parts of equal capacity ; 2nd, the introduction of the mercury into the bulb; 3rd, the graduation,

Division of the Thermometrie Tube.--As the indications of the one side, and applying the heat of a spirit-lamp to the bulb, the thermometer are exact only when the divisions of the scale air within the latter is expanded, forced up the tube, and placed on the tube correspond to equal degrees of expansion partially expelled through the funnel c, into the atmosphere. in the mercury contained in the bulb, it is of considerable The tube is then allowed to cool, and held in a vertical posiimportance that the scale should be graduated so as to indicate tion; the remaining portion of heated air in the tube contracts

; equal capacities in the interior of the tube. If the tube be and the atmospheric pressure forces the mercury into the bulb perfectly cylindrical and of the same diameter throughout, it p, however small the diameter of the capillary tube may be. would be sufficient, for the purpose of obtaining equal capaci. The introduction of tho mercury into the bulb or reservoir

, ties, to divide the length of the tube into equal parts. But however, soon ceases : because the remaining air, by the the diameters of glass iubes being generally greater at one end diminution of its volume, acquires a tension capable of resist

. than at the other, it follows that equal capacities of the tube ing the pressure of the atinosphere and of the column of mer: should be represented on the scale by unequal lengths. In cury in the tube. By heating the bulb, and then allowing it order to determine these lengths, a column of mercury of about to cool as before, a new quantity of mercury is introduced into one inch in height is introduced into the tube without the it; and this process is continued until only a very small bulb; and being kept at the same temperature, it is made to quantity of air remains in it. In order to expel this, the move in the tube in such a manner that at every change of its mercury in the bulb is heated to ebuilition; by this means the position the column advances by a quantity exactly equal to vapour of the mercury is disengaged and forced out of the its length; that is, one of the extremities of the column comes tube, carrying along with it all the air and humidity which of a divided rule or scale, to which the tube is applied at every mercury in a pure and dry state, the funnel c is removed, and change of position, we can determine, to the two or three the extremity

to which it was attached is hermetically sealed. hundredih part of an inch, the length of the tube occupied by Before this is done, however, care must be taken to expel onethe column of inercury: If it should happen that this length half or two-thirds of the mercury in the tube ; otherwise, this remains invariable, it is plain that the capacity of the tube is mercury would expand and break the thermometer. The the same throughout its entire length; but if it varies, and proper quantity of mercury to be expelled will, of cour goes on decreasing, for instance, this proves that the interior depend on the use which is to be made of the instrument; for diameter of the tube is increasing. If by this procere it is the higher the temperatures are which it is intended to Cam

sure, the greater must the quantity be which is to be expelled. per, through which the stem passes. The vessel being halfCare must also be taken that at the moment of sealing the filled with water, is placed on a furnace and heated to ebullitube, the bulb D must be heated so that the mercury will tion. The steam fills the tin tube and envelopes the therexpand and rise to the top of the tube. Thus no air will be mometer, in which the mercury rises at first, and then becomes lelt in the thermometer; for, were any air allowed to remain, stationary at a certain point. At this point, which is that of it would be compressed when the mercury rose in the tube, the level of the mercury in the stem, another small horizontal and would occasion the instrument to break in pieces. mark is made as before; this is the boiling point, marked 100°

Graduation of the Thermometer. -Having introduced the on the Centigrade thermometer, and 212° on Fahrenheit's thermercury into the thermometer and hermetically sealed it, the instrument must now be graduated, that is, prepared, by the

Fig. 160. construction of a scale, for measuring the variations of temperature. For this purpose, the first thing to be done, is to fix on the stem or tube two invariable points which shall represent temperatures easily reproduced and always the same. Now, experience has demonstrated that the temperature of melting ice is always the same, whatever may be the source of the heat by which it is produced ; and that distilled water, under the same atmospheric pressure and in a vessel of the same material, always enters into a state of ebullition at the same temperature. Accordingly, the first fixed point of the thermometer, which is the zero point, or that of 0° of the scale in the Centigrade thermometer used in France, but the point of 32° of the scale in Fahrenheit's thermometer used in this country, is that of the temperature of melting ice, commonly called the temperature of freezing or the freezing point, and the second fixed point, which is the point of 100° of the scale in the Centigrade thermometer, but the point of 212° of the scale in Fahrenheits thermometer, is that of the temperature of the boiling of distilled water, commonly called the boiling point; the ebullition being supposed to take place in a metallic vessel, while the atmospheric pressure is 769 millimetres or 29.922 inches for the Centigrade thermometer, or 30 inches for Fahrenheit's thermometer. The graduation of the thermometer, therefore, requires three operations : the deterraination of the freezing point of water, the determination of the boiling point of water, and the construction of the scale.

1st. In order to determine the freezing point, a vessel is filled with pounded ice or snow, fig. 159; and it is furnished

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Fig. 159.

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mometer. To this apparatus, M. Regnault adds a second tin tube surrounding the first, and permitting the steam to pass between it and the first, so as to preserve the interior tin-tube from being cooled by contact with the atmospheric air. The determination of the boiling point of the scale requires that the height of the barometer should be fixed, say at 30 inches, during the experiment; because, as we shall soon see, when this height is greater or less than 30 inches, water boiis at a temperature above or below the degree marked on the scale.

Nevertheless, we can obtain the exact point marked on the scale, whatever be the atmospheric pressure, by making a correction pointed out by M. Biot... This philosopher found that when the mercury rose or fell 27 millimetres (that is, 1.063 inches), the temperature rose or fell 1° Centigrade (that is, 10.8 Fahrenheit); consequently, if the height of the barometer is, for example, 778 millimetres, or 18 metres, which is of 27, above 760 metres, water boils at 1000%; this therefore is the point on the Centigrade thermometer, at which water boils under the increased pressure. Measured by inches and referred to Fahrenheit's scale, if the height of the barometer is 30.631 inches, or •709 of an inch, which is fof_1.063 inches, above

29.922 inches, water boils at 2130-2 un Fahrenheit's thermowith a hole at the bottom, to allow the water which proceeds meter. from the melting ice to escape. The bulb or reservoir of the M. Gay-Lussac having observed that water boiled in a glass thermometer, and part of the stem, are immersed in this ice for vessel at a temperature somewhat higher than in a metallic about a quarter of an hour; the column of mercury at first vessel, and that the temperature of ebullition was raised by sinks rapidly

, and then remains s'ationary at a certain point. the salts which water hela in solution, it was found necessary, At this point, which is that of the level of the mercury in the till very recently, that, in order to determine the boiling point stem, a small horizontal mark is made on a slip of paper of thermometers, a metallic vessel and distilled water must be previously pasted on the stem; this is the freezing point, employed. These two conditions, however, have been rendered marked 0° on the Centigrade thermometer, and 32° on Fahren- unnecessary by the discovery of M. Rudberg, a Swedish heit's thermometer. 2nd. In order to determine the boiling philosopher. that the nature of the vessel, and the sales held Dur : the apparatus r« resented in fig. 160 is employed. It in solution in water, have an influence on the temperature of consists of th vessel, in which is inserted a tube with two boiling water, but not on the temperature of the steam which is eral pipes, to permit the steam to escape. In the interior of produced ; that is

, that if the water be raised to a temperature the tube is placed the thermometer, supported by a cork stop-labove the boiling point marked

on any scale, the temperatur

of the steam which is disengaged from this water is still only | Centigrade is equal to 188 or f of 1° Fahrenheit; and conversely, that of the boiling point on that scale, viz, 100° on the Centi. 1° Fahrenheit is equal to 188 or 3 of 1o Centigrade. Now io grade and 212° on Fahrenheit's thermometer, so long as the convert a certain number of degrees of Fahrenheit's scale, into atmospheric pressure remains at the mean height, when these the corresponding number of degrees of the Centigrade scale, points were determined. It follows, then, that in order to we must first subtract 32 from the given number, in order to ascertain the second fixed point of the thermometer, it is not reckon the two kinds of degrees from the same point of the necessary to use either distilled water or a metallic vessel. It stem, and then multiply the remainder by 6 ; thus, 95° Fah. is sufficient, when the pressure of the atmosphere is at its mean renheit is equal to 350 Centigrade ; for 95 — 32 = 63, and 63 value, or when corrected as above-mentioned, that the thermo- X=35. Conversely, to convert a certain number of degrees meter be immersed wholly in steam and not in boiling water. of the Centigrade scale into the corresponding number of Besides, even in making use of distillel water, the bulb of the degrees of Fahrenheit's scale, we must multiply the given thermometer must not be immersed in it when boiling; for it number by S, and to the product add 32; thus 35o Centigrade is only at the surface that it is really at the proper temperature is equal to 95° Fahrenheit; for 35 X !=63; and 63 + 32 of the boiling point, the temperature increasing below this = 95. towards the bottom, on account of the excess of pressure arising from the superincumbent strata of water.

3rd. Construction of the Centigrade Scale. When the two LESSONS IN CHEMISTRY.–No. XXVIII. fixed points are obtained, as above described, the interval or ALTHOUOU the tests we employed in the course of the preced. space between them is divided into 100 parts of equal capacity, ing lesson sufficed for the purpose of indicating the existence which are called degrees, and these divisions are cxtended on of lead under that one series of conditions, there are several the scale beyond the two fixed points above and below them, other tests of importance so great that they cannot be passed as shown in fig. 157. In order to mark the degrees, it is suficient to divide the interval from 0° to 100° into 100 equal

over without comment. parts, when the rube of the thermometer is of the same dia. pose, if dissolved in water; and when added to a portion of our

Either chromate or bichromate of potash will serve our purmeter throughout; but as this condition is never rigorously lead solution, will determine a yellow precipitate. This is a satisfied, it is necessary to employ the divisions into parts very characteristic and a very delicate test. of equal capacity which were first marked on the tube, as already described. For this purpose, we count the number of lead in solution, as you will not fail to observe on trying the

Solution of iodide of potassium is also a delicate test for these divisions contained between the two fixed points, and dividing this number by 100, we have the number of divisions

experiment. which are equivalent to one degree; and we determine succes either free, or more generally united as we find it in carbonates.

The next test we shall bring into operation is carbonic acid, sively, reckoning from zero, the exact position of each degree. In our preceding lesson it was pointed out that, notwithstandIn order to distinguish the temperatures below zero, or 09, from ing the purity of the water used for the purpose of effecting those above, we prefix to the number expressing these degrees our solution of acetate of lead, the solution became after the the sign minus, that is, —. Thus, 15° below zero is indicated lapse of a short period of exposure to atmospheric air more by -- 16o.

In accurate thermometers, the scale is graduated on the or less turbid. This turbidity is due to the presence of carglass stem itself. In this manner, it cannot be displaced, and

bonic acid in the atmosphere. If, instead of merely exposing its length remains sensibly the same, glass being capable of blows through

a portion by means of a tube, as represented in

a solution of acetate of lead to the air, the operator repeatedly very little expansion. In order to obtain permanent marks our diagram, then the whiteness is much increased; thus upon glass, the thermometric stem is covered, when warm, demonstrating amongst other things the existence of carbonic with a slight coat of varnish; the marks of the scale, with their acid in the breath we expel from the lungs, fig. 24. corresponding figures, are then made on the varnish with a fine steel point; the stem is, lastly, exposed to the vapour of

Fig. 21. hydrofluoric acid, which possesses the property of acting on glass, and engraving the parts from which the varnish has been removed.

Different Thermometric Scales.- In the graduation of thermometers, three different scales are in use : the Centigrade ; the scale of Reaumur; and the scale of Fahrenheit. The Centigrade scale is that whose construction has just been explained; it is most generally used in France. It was invented by Celsius, a Swedish philosopher, who died in 1744. In the second scale, adopted in 1731 by Reaumur, a French philosopher, the two fixed points are still the temperature of melting ice and that of boiling water; but their interval is divided into 80 degrees; that is, 80 degrees of the scale of Reaumur are equal to 100 degrees of the Centigrade scale. Accordingly, 1° Reaumur is equal to 10 or of 1° Centigrade ; and conversely, 1o Centigrade is equal to 15% or of 10 Reaumur. Consequently, in order to convert a number of degrees of the sale of Reaumur into the corresponding number of degrees of the Centigrade scale, this number must be multiplied by #; thus, 20° Reaumur are equal to 25° Centigrade, for 200 x = 25°. Again, in order to convert a number of degrees of the Centigrade scale into the corresponding number of degrees of the scale of Reaumur, this number must be multiplied by l; thus, 25° Centigrade are equal to 200 Reaumur, for 250 x 1 = 20°

Fahrenheit, of Dantzic, adopted, in 1714, a thermometric scale, of which the use has extended over Holland, England, and North America. The upper point of this scale still cor. responds to the temperature of boiling water, but the zero point corresponds to the degree of cold obtained by mixing equal paris of pounded sal ammoniac and snow, and the interval between these two fixed points is dirided into 212 degrees. The thermometer of Fahrenheit placed in melting ice marks 323 on the scale; consequently, 100° Centigrade are equal to

The chemical study of this carbonate of lead is one of grest iső- Fahrenheit; for, 212° --32° = 180o. Accordingly, 1• ' importance; for the white crust which occasionally forms on

line

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