3 8 POPULAR SCIENCE MONTHLY. 



perature 48 C. ( 54.4 F.), and solid carbonic acid and ether, that 

 is capable of lowering the temperature to 100 C. ( 148 F.). 

 But even with the latter mixture it is not possible to reach the critical 

 temperature of oxygen or that of nitrogen. How, then, is it pos- 

 sible to reach these extremely low temperatures? 



In order to answer this question it will be necessary to take into 

 consideration certain temperature changes that are observed when 

 solids are melted and liquids are boiled, as well as when gases are 

 liquefied and liquids are frozen. When heat is applied to a mass of 

 ice at its melting point it melts and forms a mass of water having 

 the same temperature. Heat disappears in the operation. It is 

 stored up in the water. This disappearance of heat that accom- 

 panies the melting of ice can be shown in a very striking way by 

 mixing a certain weight of ice with the same weight of water that 

 has been heated to 80 C. (176 F.). The ice will melt and all the 

 water obtained will be found to have the temperature of the melting 

 ice that is, C. (32 F.). The water of 80 C. is thus cooled 

 down to by the melting of the ice. Again, when heat is applied 

 to water its temperature rises until the boiling point is reached. 

 Then it is converted into vapor, but this vapor has the temperature 

 of the boiling water. During the process of boiling there is no rise 

 in the temperature of the water or of the vapor. Heat disappears, 

 therefore, or is used up in the process of vaporization. Similar phe- 

 nomena are observed whenever a solid is melted or a liquid is boiled. 

 When, however, a gas is liquefied it gives up again the heat that is 

 absorbed by it when it is formed from a liquid; and so also when 

 a liquid solidifies it gives up the heat it absorbs when it is formed 

 from a solid. 



But it is not necessary that a gas should be converted into a 

 liquid in order that it should give up heat. Whenever it is com- 

 pressed it becomes warmer. Some of the heat stored up in it is, 

 as it were, squeezed out of it. Conversely, whenever a gas expands, 

 it takes up heat and, of course, surrounding objects from which 

 the heat is taken become colder. Now, it is a comparatively simple 

 matter to compress air. Every wheelman knows that, and he also 

 knows that the process causes a rise in temperature; at least he knows it 

 if he uses a small hand pump. With large pumps run by steam any 

 desired pressure can be reached. This is simply a question of secur- 

 ing the proper engines, and vessels sufficiently strong to stand the 

 pressure. It has already been pointed out that several gases are 

 now liquefied on the large scale by means of pressure. It is to be 

 noted that low temperatures can be produced by converting certain 

 gases, such as ammonia and carbonic acid, int?> liquids, and by com- 

 pressing certain gases, as, for example, air. When liquefied gases 



