47 2 PHYSIOLOGICAL PHYSICS. [Chap, xxxvn. 



or about 174 F. The same amount of heat is required 

 to convert 1 pound of ice into water at C., as 

 would raise the temperature of 1 pound of water 

 from 0C. to 79C. ? or from 32 F. to 174 F. The 

 latent heat of water is expressed by the figure 79, or 

 142 (174 - 32), according to the scale employed. 



The same rule applies to every solid body. Its 

 liquefaction involves the disappearance of heat ; and 

 the liquefaction may be accomplished without eleva- 

 tion of temperature. Ice, however, requires more 

 heat for its liquefaction than other bodies. While 

 the latent heat of water is about 79, that of tin is 

 about 14, of lead over 5, of sulphur over 9. 



Freezing- mixtures exemplify very well the 

 facts that have been stated. When ice is mixed with 

 salt in the proportion of two parts of the former 

 to one of the latter, the ice is rapidly melted, the 

 rapid thaw involving a rapid disappearance of heat. 

 If the mixture surrounds a liquid at the ordinary 

 temperature, and if care is taken to prevent the ice 

 obtaining the necessary heat from other sources, the 

 heat will be abstracted from the liquid, and in a short 

 time its temperature will be so reduced that it will 

 become frozen. The temperature obtained by the 

 mixture of ice and salt, or snow and salt, is about 

 zero Fahrenheit. Other freezing mixtures are snow 

 and crystallised chloride of calcium (3 to 4), nitrate of 

 ammonia and water (equal parts), sulphate of soda 

 and hydrochloric acid (8 to 5). 



EbuISitiosa is the condition in which, by heating, 

 bubbles of vapour are formed in the interior of a 

 liquid which pass to the surface, and there become 

 disengaged. In water, being heated in a flask over a 

 lamp, bubbles may be seen to become disengaged 

 from the bottom of the flask, on the outside of which 

 the flame is playing ; but they do not become disen- 

 gaged at the surface. They disappear as they pass 



