76 Chemistry. [Lecture 26. 



weigh it again filled with air, and lastly filled 

 with water. This will give you the weight of 

 steam compared with that of water, which is in 

 the proportion of about 1 664 to 1 . 



The point at which a fluid is converted into 

 vapour depends' upon a certain degree of heat, 

 and this we call the vaporific or boiling point, 

 which differs widely in different bodies. The 

 terms volatility and Jixedness are only terms of 

 comparison. Thus, when we say a body is vo- 

 latile, we only mean that it requires less heat to 

 convert it into vapour than most other bodies ; 

 and by a contrary reason we define bodies fixed. 

 The vaporific point is always the same in the same 

 bodies. It agrees in this with the effect of heat 

 in producing fluidity, but the vaporific point is 

 remarkably influenced by mechanical pressure. 

 The greater the pressure, the greater the degree 

 of heat necessary to convert the fluid into va- 

 pour. Fahrenheit, by the barometer, observed 

 that according to the greater or less pressure of 

 the atmosphere, a fluid required a greater or less 

 degree of heat to convert it into vapour; he 

 marked the vaporific point of water on his ther- 

 mometer at the mean height 212. If water, how- 

 ever, can be prevented from going off in steam, 

 as it may by means of a particular contrivance, it 

 will acquire a degree of heat equal to that of 

 metals when red-hot. The machine for this pur- 

 pose is called Papin's Digester, and is a copper 

 vessel half filled with water, the head screwed in 



