PYRONOMICS. 103 
' From the experiments upon the specific heat of bodies, many remarkable 
results have been ascertained, among which not the least important is the 
law discovered by Dulong and Petit, that the specific heat of bodies is 
inversely as their atomic weights, or in other words, that the product of the 
specific heat and the atomic weight of certain bodies is always a constant 
quantity. There may be here and there slight differences, yet the products 
fall within narrow limits, being for elementary bodies between 37.849 and 
42.703. The specific heat of a body experiences some change with its 
density. With respect to the specific heat of compound bodies, Avogadro, 
Neumann, and Regnault have determined, that in all such bodies of equal 
atomic and similar chemical composition, the above law equally holds 
good. : 
The specific heat of gases has been investigated by De la Roche and 
Berard. The apparatus used by them in their experiments is represented 
on pl. 19, fig. 46. The vessel, a, filled with air, has an air-tight cover, 
through which a perpendicular tube is raised, opening into a vessel, A, filled 
with water, so that the water can enter the vessel a. Through the 
air-tight cover of the vessel A, there passes into the water a tube open at 
both ends, so that when the water passes out of A, bubbles of air can enter 
A. through the lower end of the tube. From the vessel, a, pass two tubes, 
afterwards uniting into one,to the balloon, c. One of these tubes reaches 
nearly to the bottom of a, and is closed by a cock; through the other pass 
the upper portions of air from atoc. In ¢ is suspended a bladder, J, filled 
with gas to be examined, from which the gas passes by the pressure of the 
air compressed in c, through the tube, m, into the worm of the calorimeter, 
s. It is previously heated in its passage through e, by the steam rising 
from boiling water. The gas, after passing through the calorimeter, is 
conducted through the tubes 2 and p, into the empty bladder, e, placed in 
the balloon, D. From this balloon there is conducted a tube. q, entering 
the vessel, d (filled with water), by two branches, one of which, provided 
with a cock, Jeads to the upper part of the vessel, the other goes nearly to 
the bottom. When the air passes through this latter arm from D to d, the 
water flows from d through a cock. If the bladder, /, be empty, and c 
filled with gas, then @ must be filled with water, and d with air; all the 
cocks hitherto open are closed, and those closed opened. The air in D 
and d is immediately compressed by the water coming from B, and the gas 
driven out of the bladder, e, through the tubes p and v, towards the heating 
part, e, thence to the calorimeter, from whose worm it reaches the bladder. 
i, through the tubes zn, w, and m; the air from c is forced into a, and the 
water in a flows out through the cock, h. If the bladder, J, be filled afresh 
with gas, the circuit begins anew. One thermometer indicates the 
temperature with which the gas enters the calorimeter, a second its 
temperature at the exit, and a third the temperature of the water in the 
calorimeter. A screen separates the calorimeter from the rest of the 
apparatus, to keep off accidental changes of temperature. 
The heated gas passing through the worm of the calorimete: 
communicates to the surrounding water a certain amount of heat, so that 
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