_--We have already seen that when a salt of any kind dissolves
in water, heat is absorbed, and becomes latent; in other words, cold is
produced. I will describe a remarkable example or experiment, well
illustrating this fact. If you take some Glauber's salt, crystallised
sulphate of soda, and mix it with some hydrochloric acid (or spirits of
salt), then so rapidly will the solution proceed, and consequently so
great will be the demand for heat, that if a vessel containing water be
put in amongst the dissolving salt, the heat residing in that vessel and
its water will be rapidly extracted, and the water will freeze. As
regards solubility, some salts and substances are much more quickly and
easily dissolved than others. We are generally accustomed to think that
to dissolve a substance quickly we cannot do better than build a fire
under the containing vessel, and heat the liquid. This is often the
correct method of proceeding, but not always. Thus it would mean simply
loss of fuel, and so waste of heat, to do this in dissolving ordinary
table salt or rock salt in water, for salt is as soluble in cold water
as in hot.
Pages:
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69