CHAPTERS FOR STUDENTS. 
723 
15. Such experiments as those just described, tell us, that to make the 
temperature of water rise or fall through the same'number of degrees on a 
thermometric scale, it requires the addition or subtraction of thirty times as 
much heat as is required to effect the same change in an equal weight of 
mercury ; or conversely, the mercury requires ^th of the heat necessary for 
the water. The numerical expression of this fact would give us, making 
water = 1, mercury = ’033. These numbers are their respective specific 
heats. Nothing as to absolute amount or quantity of heat is to be inferred 
from the use of these numbers ; they merely indicate the relative powers of 
the two bodies of, as it were, assimilating heat. 
16. The specific heat of a body is ascertained by one or other of several 
methods. 
a. By the process of mixture above described, in connection with the ex¬ 
ample of water and mercury. 
b. By observing the different degrees of rapidity with which weighed 
quantities cool down from some chosen temperature to a lower one. 
c. By ascertaining the relative quantities of ice melted when equal weights 
of the bodies experimented upon are allowed to yield their heat to it in fall¬ 
ing from the chosen temperature to 32° F. 
17. In experimenting upon the different elementary bodies, an interesting 
fact comes to light. It is found that the greater the atomic weight of an ele¬ 
ment, the smaller is its specific heat, and vice versa ; they are inversely pro¬ 
portional to each other. The consequence is, that if the specific heats of 
the elements be multiplied by their atomic weights, we get a series of products 
which are very nearly the same. For example, the atomic weight of po¬ 
tassium is 39T, its specific heat T695, and the product of these two numbers 
is 6’5. The atomic weight of sodium is 23, its specific heat ‘2934, and the 
product of the two is 6'748. # 
18. It follows from what has just been said, that the atoms of all elemen¬ 
tary bodies have associated with them the same amount of heat. 
CHEMICAL NOTES TO THE PHARMACOPOEIA {continued). 
The former of these two observations is merely a reiteration of the law of 
multiple proportions. If hydrogen combines with a fixed quantity of another 
element, it can only do so at the rate cf 1 gram at a time ; chlorine in the 
same way can unite with other bodies only in parcels of 35| grams at a time. 
It follows, then, that the measures of different elements which combine must be 
the measures which contain these weights, viz., once, twice, or more times 
llT litres. 
To understand the second statement, it is only necessary to think of a defi¬ 
nition for specific gravity. The specific gravities of any number of substances 
may be said to be the comparative weights of equal volumes of those substances. 
Now, 1, 35'5, 16, 80, and 127 are the respective weights of one and the same 
measure of the gases enumerated, so that these numbers stand for the specific 
gravities of the same, one amongst them, hydrogen, being set up as the unit for 
comparison. 
Thus far, the symbols have been taken to represent a certain number of 
grams of each of the different elements ; but for the gram any other unit of 
weight may be substituted. Thus, II may be used to express 1 grain or 
1 pound of hydrogen ; in such case, Cl will stand for 35 e grains or pounds, O 
for 16 grains or pounds, and similarly with the rest of the elements. 
The quantities actually employed iu an experiment may be one-half or a 
* See a paper by the Author, Phartn. Journ, N.S. Yol. IX. p. 532. 
