697 



SPECIFIC HEAT. 



SPECIFIC HEAT. 



100 Cubic Inches weigh 



ice, which reduces the gas to the French standard ; more gas is 

 introduced to equalise the pressure, the stop-cock is then closed, the 

 globe withdrawn and wiped carefully with a damp cloth to prevent the 

 surface from becoming electric, and it is then attached to the scale-pan. 

 Two hours are allowed to elapse before it is weighed, in order that it 

 may acquire 'the temperature of the surrounding air and get rid of 

 the currents about it. The weight is then accurately noted, the 

 globe again plunged in ice, the gas removed by means of the air 

 pump, and the elasticity of the remaining portion in the globe 

 measured by the pump gauge. The empty globe is again weighed as 

 before, and the difference of the two weights will give the weight of a 

 bulk of gas the elasticity of which is equal to that of the atmosphere 

 aa marked by the height of the barometer //' diminished by the 

 elasticity A of the remaining gas as measured by the gauge. If the 

 capacity of the globe has been previously determined with accuracy, 

 the corrected weight of the gas will be obtained by the following 

 proportion : 



The standard The observed The observed Corrected 

 pressure. pressure. weight. weight. 



As // : H'-h : : W : W 



The following table has been calculated by Professor Miller from 

 Regnault's experiments, and reduced to the English standard tempe- 

 rature and pressure. 



At 32' Fahr. At 60 Fahr. Sp. gr. 



Grains. Grains. Air=l. 



Air .... 32-698 30-934 1-0000 



Oxygen .... 36-153 34-203 1-1056 



Nitrogen . . . 31-762 30-119 0-9713 



Hydrogen . . . J'205 2-143 0-0692 



Carbonic acid . . 50-000 47 SOJ 1-5290 



For further details on this subject, and for the method of deter- 

 mining the specific gravity of vapours, we must refer to Professor 

 Miller's ' Elements of Chemistry,' Part I. 2nd edition, 1860. In the 

 appendix to the third part of that work will be found tables of specific 

 gravities corresponding to degrees of Baume's hydrometer, and also of 

 Twaddell's hydrometer, the latter being converted into their corres- 

 ponding specific gravities by multiplying them by 5 and adding 

 1000. The specific gravities of the various solids, liquids and gases 

 are given under their respective heads in this Cyclopaedia, also 

 in works on Chemistry. Copious tables are also published in a 

 separate form. As a sample of such tables, a list of the specific 

 gravities of the metals is given at the end of this article. A 

 number of instruments on the principle of the HYDROMETER are sold 

 in the shops, such as the Aremneter, the Lactometer for testing the 

 quality of milk, the Saccharwnettr for enabling the brewer to form an 

 estimate of the quality of his sweet-wort, and some others. The 

 liarometrical Agnometer consists of a tall siphon tube, inverted and 

 mounted on a pedestal with a graduated scale between the two limbs. 

 It was intended to compare the specific gravities of immiscible liquids ; 

 thus 1 inch of mercury in one limb will balance about 13J inches in 

 the other limb. Erewster's Staktometer or Drop-measurer is an instru- 

 ment for measuring specific gravities by the size of the drops which 

 exude from a small orifice. The instrument is formed like a pipette, 

 and is filled by the action of the mouth with distilled water, and the 

 number of drops which escape between an upper and a lower level are 

 counted, and serve as a standard. The instrument may now be filled, 

 say with proof spirit, and the drops similarly counted. In one instru- 

 ment the number of drops of water was 724, while the number with 

 proof spirit was 2117, thus indicating that the drop of water was 

 about three times the size of the drop of proof spirit. What are 

 called tpecific gravity beadt or hollow beads of different sizes with 

 projecting tails, and marked with certain numbers, are used to show 

 roughly the density of a liquid. A number of these beads being 

 thrown into it, those which sink or swim are of no account, but those 

 which remain just suspended indicate the specific gravity. 



temperature. So also if these two bodies be removed from a warm to 

 a cold atmosphere the oil will cool much quicker than the water. In 

 comparing various bodies with water it will be found that they all 

 vary in their rates of heating and cooling. Taking water as the 

 standard of comparison, the thermal unit is the quantity of heat 

 required to raise' 1 Ib. of pure water from 32 to 33. And in general 

 the quantity of heat required to raise 1 Ib. of any other body from 32" 

 to 33 is called its specific heat. When the quantity of heat required 

 to raise the temperature of a body one degree is uniform throughout, 

 or in a very large portion of the thermometric scale, the specific heat 

 of such body is said to be uniform. Where such is not the case, it is 

 said to be variable, and is in general found to increase with the 

 temperature. 



Three methods are adopted in color imetry, as it is called, to distin- 

 guish it from thermometry, or the measuring of sensible or apparent 

 heat. First, by measuring the heat by the quantity of ice which the 

 body in question liquefies ; Secondly, by calculating it by the method 

 of mixtures ; Thirdly, by observing the rate at which heated bodies 

 cool. 



By the first method the calorimeter of Messrs. Lavoisier and Laplace 

 is used. It consists of two similar metallic vessels, one smaller than 



and contained in the other, so as to leave a space c c. From this space 

 proceeds a discharge-pipe with a stop-cock E. From the bottom of the 

 inner vessel also proceeds a pipe D; it passes air-tight through the 

 outer vessel, and is furnished with a stop-cock. There is a third 

 vessel, A, contained within the second, and the space B B is also filled 

 with pounded ice. The vessel A is furnished with a cover, and there 

 is a large tray-like cover over the whole apparatus. This is also filled 

 with pounded ice. Now supposing such an apparatus to be introduced 

 into a room at a temperature of 40, it is evident that the ice in c will 

 slowly melt, and retain B at the constant temperature of 32. Now 

 if we introduce the body whose specific heat is to be determined into A 

 at any given temperature above 32, it would cool down to that tem- 

 perature, and in doing so will melt a quantity of ice, the water from 

 which will be collected in the bottle below D. If this quantity of water 

 be divided by the number of degrees through which the body in A has 

 fallen, the quantity of ice dissolved by the heat corresponding to 1 

 will be found. This being divided by the weight of the body in A in 

 pounds, the weight of ice dissolved by the heat which would raise 

 1 Ib. of the body 1 will be determined. In this way it is found that 

 the heat necessary to raise 1 Ib. of water 1, is that which would dis- 

 solve tho 142'65th part of 1 Ib. of ice. For other bodies we get the 

 following rule : Multiply the weight of ice dissolved by 142'b'5, and 

 multiply the weight of the body which dissolves the ice, by the num- 

 ber of degrees of temperature which it loses, and divide the former 

 product by the latter, when the quotient will be the specific heat of 

 the body. 



By the method of mixtures a mean temperature is obtamed,wheu 

 equal weights of the same body are mingled together ; but when 

 different fluids are mixed the result is different. For example, 

 1 Ib. of mercury at 40 agitated with 1 Ib. of water at 156, gives a 

 mixture = 152'3. In this case the water loses 37, and the mercury 

 gains 112-3. Hence 112'3 : 3'7 1:1: 0'033, or, in other words, 

 water being 1000, the specific heat of mercury is only 33 ; 1 Ib. of 

 water absorbing 30 times more heat than 1 Ib. of mercury. 



By the third method equal and similar volumes of two bodies are 

 raised to the same temperature, and allowed to cool under similar 

 circumstances. By observing the intervals of time required for equal 

 volumes to fall 1, we get the ratio of the quantities of heat which 

 they lose. The quantities for equal weights may be inferred from the 

 specific gravities of the bodies, and in this way the specific heats can 

 be arrived at. 



The specific heat of bodies diminishes with an increase of density, 

 so that mere mechanical compression will raise the temperature of 

 many bodies, and even make some of the metals very hot ; iron, it is 

 said, incandescent. The sudden compression of aeriform bodies is 



