864 
CHAPTERS FOR STUDENTS. 
BY WILLIAM A. TILDEN, B.SC. LOND., 
DEMONSTRATOR OP PRACTICAL CHEMISTRY TO THE PHARMACEUTICAL SOCIETY. 
HEAT (continued). 
19. Possibly the following considerations will make the assertion of the last 
paragraph more clear. Suppose we find that 1 gram of the metal lithium, in the 
process of having its temperature raised from 32° F. to 212° F., consumes 9408 
units of heat, and that 1 gram of the metal sodium, in going through the ope¬ 
ration, consumes 2934 of the same units. To raise the temperature of 7 grams 
(one atom) of lithium to the same degree, would, of course, require seven times 
as much heat as would be required for 1 gram, or, 9408 X 7 = 65,856 units. 
And so also to raise the temperature of 23 grams (one atom) of sodium to the 
same degree would require twenty-three times as much heat as would be neces¬ 
sary for 1 gram, or, 2934 x 23 = 67,482 units. These two quantities, 65,856 
and 67,482, are very nearly equal, and this establishes the proposition. 
20. Any influence which changes the density of a body, that is, which alters the 
distance between its particles, causes a change in the specific heat of the body. 
Elevation of temperature causes expansion, and, therefore, increase of specific 
heat; depression of temperature causes contraction, and, therefore, decrease of 
specific heat. Compression by mechanical means also causes diminution of specific 
heat. When a solid body is melted a sudden increase of specific heat is usually 
observable, but when a liquid is converted into a gas the specific heat again 
diminishes. The specific heat of water is twice that of ice, and more than twice 
that of steam. These facts help to explain the slight differences in the observed 
atomic heats. 
21. Ice is now an article of manufacture. It is prepared on a small scale by 
immersing a thin metallic vessel containing the liquid to be frozen in a mixture 
of various soluble salts mixed with water or other liquid. As the salts dissolve 
in the water, that which was previously a solid mass is converted into a liquid ; 
whilst this change is going on heat is passing into the saline liquid from every¬ 
thing in contact with it; the containing-vessel with the water are most easily 
accessible, and are, therefore, rapidly deprived of part of their heat; if the pro¬ 
cess is successful, so much heat is removed from them to the cooling mixture, 
that the temperature is reduced below the freezing-point, and ice is the result. 
On the large scale, ice is made by causing some very volatile liquid to evapo¬ 
rate rapidly, and so arranging the apparatus that the evaporating liquid may 
be in contact with the vessel containing the water whose temperature it is de¬ 
sired to reduce. The accompanying drawing represents Carre’s ice-machine. 
It consists of a still, containing strong so¬ 
lution of ammonia, connected, gas-tight, 
with a cylindrical box, which serves as the 
condenser. Heat applied to the body of 
the still causes most of the ammoniacal gas 
to be driven over, and condense with a 
little water in the receiver, which is kept 
cool. As soon as the whole of the am¬ 
monia is expelled from a, communication 
between retort and receiver is cut off by 
means of a stop-cock, the fire is withdrawn, 
and water is introduced into the hollow 
space in the condensing box. When the 
retort is cold, it is immersed in cold water, 
the stop-cock is again opened, and the li¬ 
quefied ammonia in b boils rapidly and 
is reabsorbed in a. In doing this* the ammonia carries away so much heat 
