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CR be a perpendicular drawn from the vertex c to the plane This question was also solved by Quintin Pringle GlasABD at B, and join EA, BB, and ED.

D

E

B

Then the point E, and every other point in CE, is equally distant from the points A, B and E. For ce being the common perpendicular to each of the straight lines BA, B, and B D. (Euclid xt. Def. 3), and the edges AC, BC and DC, being equal, the bases A, B and ED, are equal (Euclid 1. 47). Also, if any point be taken in CP, its distance from the point will be the common perpendicular to three triangles, whose bases are BA, ER and ED; and these being equal, the distances of that point from A, B and ¤, are also equal (Euclid 1. 47).

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

SOLUTION OF THE CASK QUERY.
(p. 332, Fol. IV.)

Fill the three-gallon cask out of the eight-gallon cask, and pour its contents into the five-gallon cask; fill the three-gallon cask again out of the eight-gallon cask, and fill up the five-gallon cask from it; you will then have one gallon of water left in the three-galion cask and two gallons of water in the eight-gallen cask. Now, empty the five-gation into the eight-gallon cask, and you will have seven gallons of water in it: pour the one galion of water out of the three-gallon cask into the five-gallon cask, refil the three-gailon cask out of the eight-gallon cask, and pour its contents into the fire gallon cask; you will then have four gallons of water in the re-gallon cask, and four gallons of water in the eight-gallon cask; which was to be done.

This question was solved by J. Beckett, Shrewsbury; J. W. 3., Spilsby; C. Thomas, St. Austeil; F. Lawford, Luton; German, Long Acre; and a considerable number of other correspondents,

ANSWERS TO CORRESPONDENTS.

WM. R. & informed, on the best authority, that age is no hindrance to Matriculation in the University of London after a certain period, that is, after the age of 16 years, any one may matriculate at whatever age he pleases. As to the attendance in person at the apartments of the University in Somerset House, the week before the Examination-week will answer the puroose. As to the subjects, the Candidate must know what is required

Again, if DE be produced to meet AB, it will bisect it at right angles in P. For, the angle DAR is equal to the angle DEA, and the angle EAB to the angle ERA (Euclid 1.5); therefore, the remaining angles DAE and DBE are equal, hence the angles AD and BDB are also equal, since they are respec- ¦ in Chemistry, Classics, English, French, etc., according to the Regulations tively equal to the angles D A B and B (Euclid 1.5). There-inserted in the P. E. vol. 1. p. 137. fore the triangles ADF and BDF, are equal in every respect (Euclid 1. 4); whence A E is equal to RF, and the angle APD to the angle BPD Hence, à a is bisected at right angles by D (Euclid 1. Def. 10).

Now, let another perpendicular no, be drawn from the angular point n to the plane ABC, and it may in like manner be shown, that every point in this perpendicular is equally distant from the points A, B and C, and that oo, produced to meet & B, bisects it at right angles in P. But the straight lines joining D, C and 7, are in the same plane (Euclid xr. 2); hence the straight lines CE and 50, which have their extremities in this plane, are also in the same plane (Euclid. xI. 1,, therefore these perpendiculars intersect each other; and since every point in c is equidistant from the points A, B and D, and every point in Dos equidistant from the points A, B and c, their common point of intersection o, is equidistant from all the four angular, points

The calculation of the distance Do is easily effected in the following manner:

Since the straight lines A B and B E produced will bisect the opposite sides DB and AD, as Dg has been shown to do in the case of AB, we have the well-known property that is DP; and that CP, which is equal to DP, is similarly divided in o, and is equal to o F. Also, from the similar triangles DG F and DEO we have

but, Do?:

Do: Dp2:: DE: Do2 (Enc. vi. 22.) -QP (Euc. 1. 47.) (DF) (by substitution), App (by squaring), DP (by subtraction); also, D FA DF (by squaring).

Hence the above proportion becomes

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DF: Do (by substitution),
DF: Do (by alternation and dividing
the two first terms by DF);
= 37.5,

A D2

- A p2

100-25
2

or, DO✓ 37 66.12372435 inches. Therefore, taking the radius of one of the four balls from this vaine of no, we have the radius of the interior ball 112372435; or its diameter 2:217487. X PLUS Y.

JAMES GREAVES: We think his friend is mistaken as to the Latin Les sons, and that Dr. Beard performed all that he promised, and more than we did, but we sincerely thank him for his friendly mints.-W. H. TYSON Many thanks for his list of errata.-J. THOMPSON Pool) should Hulme solve our Bali question before he proposes his own.-W. GRIMMOND (AT)

DR N. Manchester: The Balances in the Ledger ought to be entered Thanks for his notes on the Algebraic questions. anew in the Journal as assets and liabilities, in order to preserve the accuracy of the system of check, which depends on the total additions of each side of the Ledger being the same as those of the Journal for any given period.

GLENITNE (Johnstone: Under consideration.-WM. B.: The specifie gravity of water being 1000, the specific gravities of other bodies are usually given in accordance with this unit, viz. one thousand; this method prevents the necessity of asing fractional parts, uniess one wishes to be very precise. With regard to water, the weight of a cubic foot of this liquid is commonly reckoned at 1000 ounces, which is only approximately true, for by the experiments on which the regulations on this point in the Act of Parliament for establishing Uniformity in Weights and Measures" were founded, it appears that "a cubic inch of distilled water weighed in air by brass weights, at the temperature of 62° Fahrenheit's thermometer, is equal to 25 158 grains, Troy weight" and that the gallon should contain 10 pounds of distilled water weighed in air at 629 Fahrenheit." Now, by proportion, we have 254-458 grains: 10 lbs. or 70,000 grains:: 1 cubic inch: 277-47384 cubic inches, or, as it is generally stated, 27-274 cubic inches, taking the But at the temperature of 39 Fahrennearest thousandth part of an inch. heit, or that of the maximum density of water, the Royal Commissioners found that a cubic inch of water weighed 253 grains. At this temperature, therefore, we have the following proportions, 1 cubic inch : 17.8 cubic inches :: 253 grains : 437,184 grains; and 1 lb. or 7000 grains: 437,94 graiss⠀⠀ 16 ozs.: 999-2777 ozs., which is very nearly 1000 os., wanting only about of an ounce, or lig drams. By the same mode of calculation, the weight of a cubic foot of water would be found to be 997 1369 ozs., which wants about 2 ounces of the 1000 ounces; so that every thing depends on the temperature of the water in accurate calculations.

A. WOOD (Manchester): There are no works on Pneumatics and Hydraulies more modern than the Lessons in the P. E.-J. R. (Hull): Would any reader inform him of a work on the Rudiments of Water-colour Drawing ?-J. WRAGG (Retford, proposes to form with others a society or class to study Botany, and requests our advice as to which system should be adopted; of course, we recommend the natural system.

E. G. SMITH (Lambeth): We do not see how we can forward his views.MARY ANNE Reading) writes us a lively and amusing letter, requesting our opinion of certain English Grammars she has studied, and asks us whether she ought to know enough to write and speak correctly. We think toned. In her letter, we have counted no less than fifteen errors in spelling the Lessons in English in the P. E. are superior to any of those she has menalone! She ought to know better. We strongly recommend to her perusal, 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 does not require it for her own use. Of the Journal she mentions we entirely disapprove. Etiquette is pronounced ǎy-tee-kétt; and again, a-gén.

ERRATA.

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ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XXIX.

(Continued from page 19.)

CALORIC.

NATURE OF HEAT; THERMOMETERS. Hypothesis on the Nature of Heat.-The name of caloric is given to the agent which produces within us the sensation of heat. This agent acts upon all bodies animate and inanimate; it melts ice, boils water, and makes iron red-hot. Numerous opinions have been formed as to the cause of heat; two only remain in the present state of science: the system of emission, and that of undulations. In the former, it is supposed that the cause of heat is a material and imponderable fluid which passes from one body to another, and whose particles are in a state of continual repulsion; and that this fluid exists in all bodies, in a state of combination with their ultimate particles, and opposed to their immediate contact. In the system of undulations, it is supposed that the cause of heat is a vibratory motion of the particles of heated bodies, which is transmitted to the particles of other bodies through the medium of a fluid wonderfully subtle and elastic, which is called ether, and in which it is propagated in the manner cf sonorous waves in the air. The hottest bodies are, consequently, those whose vibrations have the greatest amplitude and the greatest velocity, and heat is nothing else but the resultant of the vibrations of the particles. On the former hypothesis, the particles of bodies which are cooled lose their caloric; on the latter, they only lose their motion.

The progress of modern physics appears to be entirely in favour of the theory of undulations. Yet, as the theory of emission simplifies the demonstrations, it is generally preferred in the explanation of the phenomena of heat.

General Effects of Caloric.-The general action of caloric on bodies is the development among their particles of a repulsive force incessantly struggling with molecular attraction; it therefore follows that, under the influence of this agent, bodies tend at first to expand or dilate, that is to assume a larger volume; then, to change their state, that is, to pass from the solid to the liquid state, or from the liquid to the gaseous

state.

the dial; but in proportion as the rod A gets heated more and more, the index is seen rising by degrees, and thus the elongation of the rod is rendered sensible.

The cubic expansion of solids is proved by means of the ring of S'Gravezande. This name is given to a small metallic ring through which, at the ordinary temperature, a copper sphere, having nearly the same diameter, readily passes. But 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 will-a proof of its increase in volume, or of its cubic expansion, that is, expansion in all directions.

In order to demonstrate the expansion of liquids by heat, a small hollow glass ball is united to a capillary tube, and the ball with part of the tube being filled with any given liquid, we observe that, as soon as it is heated, the liquid rises in the tube, and the expansion thus observed is always much greater than that of solids. The same apparatus may be employed to prove the expansion of gases by heat. For this purpose, the ball is filled with air or any other gas, and into the tube a mercurial index of about an inch in length is introduced. When the ball is heated, by merely bringing it into contact with the hand, the index is driven towards the open extremity of the tube; and, if heat be applied long enough to the mercury, it will ultimately be expelled from the tube. Whence we conclude that, even with a slight increase of heat, gases are very expansible. In such experiments as these, as soon as the bodies are cooled, they contract and assume their original volume, when the heat is reduced to the same degree as before.

MEASURE OF TEMPERATURES.

Temperature and Thermometers.-The temperature of a body is the actual state of the sensible caloric in the body, without increase or diminution. If the quantity of sensible heat increases or diminishes, we say that the temperature is raised or lowered. The instruments which are employed to measure temperatures and ascertain their variations are called thermometers, that is (from the Greek), heat-measures. The great difference and variety of the sensations of heat excited in different individuals by the same amount of caloric in a body, prevent us from measuring this amount with any degree of certainty by means of these sensations; we are therefore obliged to have recourse to the physical effects of caloric upon certain bodies as a method of determining the degrees of heat in all others. These effects are of various kinds; and those of expansions and contractions have been generally adopted, as being the most easily observed and the most readily recorded. But heat also gives rise in bodies to electrical phenomena, by means of which temperatures can be measured, and extremely sensible thermometers have been constructed on this principle. Of all bodies, in general, liquids are the best adapted for the construction of thermometers, solids not being sufficiently expansible, and gases being too much so. The liquids exelusively adopted are mercury and alcohol; the first, because it enters into a state of ebullition at a very high temperature; and the second, because it has never been solidified, that is. Fig. 156.

All bodies are expanded by the action of caloric. The most expansible bodies are the gases; the next are the liquids; and lastly, the solids. In the case of solids, two kinds of expansion are considered: 1st, their linear expansion, that is, in the direction of one dimension only, say the length; 2nd, their cubic expansion, that in the volume or bulk of the solid. These expansions, nevertheless, take place at the same time, under the action of heat. In liquids and gases, it is only the expansion in volume that is taken into consideration.

In order to determine the linear expansion of the metals, the apparatus represented in fig 156 is employed.

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sively used. It is constructed of a capillary glass tube, to one
end of which is blown a cylindric or spherical reservoir, fig.
157.
Fig. 157.

found that the column of mercury varies considerably in its length, the tube is thrown aside, and a more regular one substituted in its place. But if the variations are small, a slip of paper is pasted on the tube lengthwise, and a mark is made with a pencil at the points successively or pied 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 thermometer 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. 158.

C

D

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 frame. 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 Thermometric Tube.--As the indications of the thermometer are exact only when the divisions of the scale placed on the tube correspond to equal degrees of expansion in the mercury contained in the bulb, it is of considerable importance that the scale should be graduated so as to indicate equal capacities in the interior of the tube. If the tube be perfectly cylindrical and of the same diameter throughout, it would be sufficient, for the purpose of obtaining equal capacities, to divide the length of the tube into equal parts. But the diameters of glass tubes being generally greater at one end than at the other, it follows that equal capacities of the tube should be represented on the scale by unequal lengths. In order to determine these lengths, a column of mercury of about one inch in height is introduced into the tube without the bulb; and being kept at the same temperature, it is made to move in the tube in such a manner that at every change of its position the column advances by a quantity exactly equal to its length; that is, one of the extremities of the column comes successively and exactly into the place of the other. By means of a divided rule or scale, to which the tube is applied at every change of position, we can determine, to the two or three hundredth part of an inch, the length of the tube occupied by the column of inercury. If it should happen that this length remains invariable, it is plain that the capacity of the tube is the same throughout its entire length; but if it varies, and gots on decreasing, for instance, this proves that the interior diameter of the tube is increasing. If by this process it is

one side, and applying the heat of a spirit-lamp to the bulb, the air within the latter is expanded, forced up the tube, and partially expelled through the funnel c, into the atmosphere.

The tube is then allowed to cool, and held in a vertical posi tion; the remaining portion of heated air in the tube contracts, and the atmospheric pressure forces the mercury into the bulb D, however small the diameter of the capillary tube may be. The introduction of the mercury into the bulb or reservoir, however, soon ceases: because the remaining air, by the diminution of its volume, acquires a tension capable of resist ing the pressure of the atmosphere and of the column of mer cury in the tube. By heating the bulb, and then allowing it to cool as before, a new quantity of mercury is introduced into it; and this process is continued until only a very small In order to expel this, the quantity of air remains in it. mercury in the bulb is heated to ebullition; by this means the vapour of the mercury is disengaged and forced out of the tube, carrying along with it all the air and humidity which remained in the bulb, When the bulb is thus filled with mercury in a pure and dry state, the funnel c is removed, and the extremity to which it was attached is hermetically sealed. Before this is done, however, care must be taken to expel onehalf or two-thirds of the mercury in the tube; otherwise, this mercury would expand and break the thermometer. The proper quantity of mercury to be expelled will, of course, depend on the use which is to be made of the instrument; for the higher the temperatures are which it is intended to mea

sure, the greater must the quantity be which is to be expelled. Care must also be taken that at the moment of sealing the tube, the bulb D must be heated so that the mercury will expand and rise to the top of the tube. Thus no air will be left in the thermometer; for, were any air allowed to remain, it would be compressed when the mercury rose in the tube, and would occasion the instrument to break in pieces.

Graduation of the Thermometer. Having introduced the mercury into the thermometer and hermetically sealed it, the instrument must now be graduated, that is, prepared, by the 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 Fahrenheit's 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 determination 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

Fig. 159.

per, through which the stem passes. The vessel being halffilled with water, is placed on a furnace and heated to ebullition. The steam fills the tin tube and envelopes the thermometer, in which the mercury rises at first, and then becomes stationary at a certain point. At this point, which is that of the level of the mercury in the stem, another small horizontal mark is made as before; this is the boiling point, marked 100° on the Centigrade thermometer, and 212° on Fahrenheit's ther

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[graphic]

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 boils 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, 19.8 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 100°; 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 of 1.063 inches, above 29.922 inches, water boils at 2130.2 on Fahrenheit's thermometer.

with a hole at the bottom, to allow the water which proceeds 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 held 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 salts held p, the apparatus re resented in fig. 160 is employed. It in solution in water, have an influence on the temperature of consists of vessel, in which is inserted a tube with two boiling water, but not on the temperature of the steam which is 1eral 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-above the boiling point marked on any scale, the temperature

1

1° Fahrenheit is equal to 188 or of 1o Centigrade. Now to convert a certain number of degrees of Fahrenheit's scale, into the corresponding number of degrees of the Centigrade scale, we must first subtract 32 from the given number, in order to reckon the two kinds of degrees from the same point of the stem, and then multiply the remainder by; thus, 95° Fahrenheit is equal to 35° Centigrade; for 95 - 3263, and 63 × 35. Conversely, to convert a certain number of degrees of the Centigrade scale into the corresponding number of degrees of Fahrenheit's scale, we must multiply the given number by 3, and to the product add 32; thus 350 Centigrade is equal to 95° Fahrenheit; for 35 X=63; and 63 +32 95.

of the steam which is disengaged from this water is still only | Centigrade is equal to 188 or 2 of 1° Fahrenheit; and conversely, that of the boiling point on that scale, viz. 100o on the Centigrade and 212° on Fahrenheit's thermometer, so long as the atmospheric pressure remains at the mean height, when these points were determined. It follows, then, that in order to ascertain the second fixed point of the thermometer, it is not necessary to use either distilled water or a metallic vessel. It is sufficient, when the pressure of the atmosphere is at its mean value, or when corrected as above-mentioned, that the thermometer be immersed wholly in steam and not in boiling water. Besides, even in making use of distilled water, the bulb of the thermometer must not be immersed in it when boiling; for it is only at the surface that it is really at the proper temperature of the boiling point, the temperature increasing below this 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 fixed points are obtained, as above described, the interval or space between them is divided into 100 parts of equal capacity, which are called degrees, and these divisions are extended on the scale beyond the two fixed points above and below them, as shown in fig. 157. In order to mark the degrees, it is sufficient to divide the interval from 0° to 100° into 100 equal parts, when the tube of the thermometer is of the same diameter throughout; but as this condition is never rigorously satisfied, it is necessary to employ the divisions into parts of equal capacity which were first marked on the tube, as already described. For this purpose, we count the number of these divisions contained between the two fixed points, and dividing this number by 100, we have the number of divisions which are equivalent to one degree; and we determine succes sively, reckoning from zero, the exact position of each degree. In order to distinguish the temperatures below zero, or 0°, from those above, we prefix to the number expressing these degrees the sign minus, that is,-. Thus, 15° below zero is indicated by 15. In accurate thermometers, the scale is graduated on the glass stem itself. In this manner, it cannot be displaced, and its length remains sensibly the same, glass being capable of very little expansion. In order to obtain permanent marks upon glass, the thermometric stem is covered, when warm, with a slight coat of varnish; the marks of the scale, with their corresponding figures, are then made on the varnish with a fine steel point; the stem is, lastly, exposed to the vapour of 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 was invented by it is most generally used in France. 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, 1° Centigrade is equal to 1% or of 1° Reaumur. Consequently, in order to convert a number of degrees of the scale 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 20° ×

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 ; thus, 23 Centigrade are equal to 20° Reaumur, for 25° × 1 = 20o.

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 corresponds to the temperature of boiling water, but the zero point corresponds to the degree of cold obtained by mixing equal parts of pounded sal ammoniac and snow, and the interval between these two fixed points is divided into 212 degrees. The thermometer of Fahrenheit placed in melting ice marks 32 on the scale; consequently, 100° Centigrade are equal to 150° Fahrenheit; for, 212° 32° 180°. Accordingly, 1°

LESSONS IN CHEMISTRY.-No. XXVIII. ALTHOUGH the tests we employed in the course of the preceding lesson sufficed for the purpose of indicating the existence of lead under that one series of conditions, there are several other tests of importance so great that they cannot be passed over without comment.

Either chromate or bichromate of potash will serve our purpose, if dissolved in water; and when added to a portion of our lead solution, will determine a yellow precipitate. This is a very characteristic and a very delicate test.

Solution of iodide of potassium is also a delicate test for lead in solution, as you will not fail to observe on trying the experiment.

The next test we shall bring into operation is carbonic acid, either free, or more generally united as we find it in carbonates. In our preceding lesson it was pointed out that, notwithstanding the purity of the water used for the purpose of effecting our solution of acetate of lead, the solution became after the lapse of a short period of exposure to atmospheric air more or less turbid. This turbidity is due to the presence of carbonic acid in the atmosphere. If, instead of merely exposing a solution of acetate of lead to the air, the operator repeatedly blows through a portion by means of a tube, as represented in our diagram, then the whiteness is much increased; thus demonstrating amongst other things the existence of carbonic acid in the breath we expel from the lungs, fig. 24. Fig. 21.

The chemical study of this carbonate of lead is one of grest importance; for the white crust which occasionally forms on

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