554 HISTORY OF THERMOTICS. 



culiar 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 thermo- 

 tical theories. We are still in the dark as to 

 many of the laws which belong to this change; 

 but one of them, of great importance, nas been dis- 

 covered, 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 conversion of solids into 

 liquids, or of liquids into vapours, there is com- 

 municated to the body heat which is not indicated 

 by the thermometer. The heat is absorbed, or 

 becomes latent; and, on the other hand, on the 

 condensation of the vapour 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 

 twenty 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 temperature raised to the boiling 

 point, 212: and this temperature continued sta- 

 tionary for twenty minutes, when the whole was 

 boiled away. Dr. Black inferred from these facts 



