The Romance of Modern Chemistry

BELOW ZERO ordinary temperature, and either by utilising some inherent property of these substances, or by treating them in some special way, induce the temperature to fall, say, below the freezing-point of water. The mere bringing together of two substances may lead to either a rise or a fall of temperature. The reader may remember the reference made in a previous chapter to the fact that when sulphuric acid and water are mixed, so much heat is produced that the containing vessel becomes too hot to hold. The opposite effect is frequently observed when other substances are mixed with water. When saltpetre or sal ammoniac, for example, is stirred into water, the cooling effect is very noticeable, and by this simple method quite a considerable fall of temperature is produced. A mixture of these two salts, added to an equal weight of water at 50° Fahrenheit, brings the temperature down to 10° Fahrenheit. More marked and more persistent cooling effects are obtained, if, instead of adding salts to water, we mix them with powdered ice or snow. Any one who procures common salt and snow, stirs them up well in the proper proportions, and puts a thermometer in the mixture will see the mercury fall below zero Fahrenheit. Such a freezing mixture may be used not only for getting a low temperature in scientific experiments, but also for the equally practical, if less exalted, object of making ices. The question may very naturally be put: “Why should the mere bringing together of salt and snow result in such a marked fall of temperature ?11 The answer to this question is very closely connected with what was said in a previous chapter about the melting-point of alloys. Attention was then directed to the fact that the melting- point of any metal is lowered by the presence of another 181