696 APPENDIX A 



heat absorbed, being insensible to the thermometer, is called 

 " latent heat." If we apply heat to a block of ice the tempera- 

 ture does not rise above the freezing-point 32 F. until 

 every particle of ice has become liquid, and similarly, when 

 water is heated to the boiling-point the temperature remains 

 at 212 F. until all the water has been turned into steam. 

 The same law applies to liquefied carbonic anhydride, 

 ammonia, and sulphurous anhydride, which are the substances 

 chiefly used in refrigerating machines. Only, these liquids 

 are so extremely volatile that the change of state takes place 

 with great rapidity, and latent heat is absorbed at a corre- 

 sponding rate. Very low temperatures are obtained by 

 simply evaporating any of these liquids in the open air. 

 But if we place one of them in a confined space, say a 

 wrought-iron coil, and attach a pump to it, the suction of the 

 pump produces a partial vacuum, which accelerates the speed 

 of evaporation, and the absorption of latent heat goes on faster 

 than ever. The necessary heat is drawn from the coil and from 

 surrounding materials, and, if the coil be placed in the atmo- 

 sphere of an insulated room, heat is rapidly withdrawn from the 

 air and from any substances stored in the room ; or, on the other 

 hand, if the coil be immersed in a tank of unfreezable brine, 

 the temperature of the latter is soon reduced below the 

 freezing-point, and if it be then pumped through a series of 

 pipes or drums placed in the insulated room, it will reduce 

 the temperature of the latter and its contents to any desired 

 extent The above-mentioned volatile liquids are valuable 

 and cannot be thrown away, and we therefore transfer our 

 attention to the other or pressure end of the pump. Here 

 the reverse action takes place. When gases are compressed 

 they diminish in volume, and their latent heat is given up 

 and becomes sensible heat, which can be measured by the 

 thermometer ; and if this heat be removed by a cooling medium 

 such as cold water, the gases become liquid again, and if the 

 ends of the two coils not connected to the pump are joined 

 together, the liquid formed in the compression coil can be 

 transferred to the suction or evaporator coil and used over 

 again, and so on ad infinitum. One charge of carbonic 

 anhydride, ammonia, or sulphurous anhydride would be 

 sufficient for all time if we were able to produce machinery 

 entirely free from leakages. 



