58 



.therefore makes them firmer rather than otherwise, as is clearly demon- 

 strated when a portion of the water is removed as in fading. 



In addition to snow, ice, and ordinary water, an invisible form 

 exists in nature as vapour suspended in the atmosphere, or as steam 

 enclosed in the boilers of our engines. 



I'hese three torms of aggregation — solid, liquid and gaseous — have 



of course the same composition; but, as we know, vary in appearance 



and properties. They are easily transformed one into the other, and 



frequently exist in nature in contact with each other. Although easily 



accomplished this transformation is not so simple as, without reflection, 



we might suppose. If a thermometer be placed in contact with melting 



ice, it will always indicate the same temperature no matter what the heat 



applied to the ice may be, and furthermore, so long as any of it remains 



in contact with the resultant water, this also does not vary, but remains 



constantly at the freezing point. Since neither the ice nor the water 



have increased in temperature the heat applied to them is not indicated 



by the thermometer, and is hence called latent heat. Heat, as we 



know, can be transformed into force, and in this case it has it has been 



used to overcome the force which holds the minute particles (called 



molecules) of which the ice is composed, in their place, preventing 



them from moving past each other as they do in liquids. The heat 



necessary to do this work can be measured by applying a definite 



amount (in the shape of hot water) to a pound of ice. If we were to 



mix a volume of water at So'^C. (176'^F.) with the same weight of ice 



at its melting point, and could prevent the loss of any heat, we should 



find that after a time the ice would have disappeared, and two volumes 



of water at the freezing point would be the result; clearly showing that 



considerable heat had been rendered latent. 



Very frequent use is made of this property of water, as for example 

 in " freezing mixtures." In these the heat required to melt the ice is 

 supplied by the materials to be cooled or frozen. A convenient form 

 is that in which this material (i.e., a can of cream) is imbedded in a 

 mixture of salt and ice. Since salt is very soluble it can cause the ice 

 to melt at a much lower point than it generally does, thereby materially 

 reducing it in temperature. In the construction of the scale for his 

 thermometer Farenheit used the lowest point obtainable by this mi.x- 



