160 HISTORY OF THERMOTICS. 



is, that it changes their form, as it is often called, that is, their con- 

 dition as solid, liquid, or air. Since the term " form" is employed in 

 too many and various senses to be immediately understood when it is 

 intended to convey this peculiar meaning, I shall use, instead of it, the 

 term consistence, and shall hope to be excused, even when I apply this 

 word to gases, though I must acknowledge such phraseology to be 

 unusual. Thus there is a change of consistence when solids become 

 liquid, or liquids gaseous; and the laws of such changes must be 

 fundamental facts of our thermotical theories. "VVe are still in the 

 dark as to many of the laws which belong to this change ; but one of 

 them, of great importance, has been discovered, and to that we must 

 now proceed. 



Sect. 3. The Doctrine of Latent Heat. 



THE Doctrine of Latent Heat refers to such changes of consistence as 

 we have just spoken of. It is to this effect ; that during the conver 

 sion of solids into liquids, or of liquids into vapors, there is com 

 municated to the body heat which is not indicated by the thermo- 

 meter. The heat is absorbed, or becomes latent; and, on the other 

 hand, on the condensation of the vapor to a liquid, or the liquid to a 

 solid consistency, this heat is again given out and becomes sensible. 

 Thus a pound of ice requires tw T enty times as long a time, in a warm 

 room, to raise its temperature seven degrees, as a pound of ice-cold 

 water does. A kettle placed on a fire, in four minutes had its tem- 

 perature raised to the boiling point, 212 : and this temperature con- 

 tinued stationary for twenty minutes, when the whole was boiled 

 away. Dr. Black inferred from these facts that a large quantity of 

 heat is absorbed by the ice in becoming water, and by the water in 

 becoming steam. He reckoned from the above experiments, that ice, 

 in melting, absorbs as much heat as would raise ice-cold water through 

 140 of temperature : and that water, in evaporating, absorbs as much 

 heat as would raise it through 940. 



That snow requires a great quantity of heat to melt it ; that water 

 requires a great quantity of heat to convert it into steam ; and that 

 this heat is not indicated by a rise in the thermometer, are facts which 

 it is not difficult to observe ; but to separate these from all extraneous 

 conditions, to group the cases together, and to seize upon the general 

 law by which they are connected, was an effort of inductive insight, 

 which has been considered, and deservedly, as one. of the most striking 



