orjECTs.] INTRODUCTORY. 55 



37. Gases and Vapours. 



Air is as much a gas in the coldest winter as it is in 

 the hottest summer. But air can be liquefied by ex- 

 posing it to a very low temperature, while, at the same 

 time, it is subjected to an extremely great pressure. 

 Thus, the difference between gases like air, which are 

 condensed with extreme difficulty, and gases like 

 steam, which are condensed easily, is only one of 

 degree. Nevertheless there is a certain convenience 

 in distinguishing those gases, which, like steam, are 

 easily condensed as vapours. In what we ordinarily 

 call steam, all the water of which it is composed re- 

 mains gaseous only at and above the temperature of 

 boiling water (212 Fahrenheit). Cooled ever so 

 little below this point, most of it becomes condensed 

 into hot liquid water. However, it must be recol- 

 lected that though that particular form of gaseous 

 water which we call steam exists only at and above 

 the temperature of boiling water, yet water is capable 

 of existing in the gaseous state down to the freezing- 

 point. 



Suppose that when our boiling flask contained 

 nothing but water and steam, the mouth were stopped 

 and the lamp removed. Then, so long as the tem- 

 perature of the whole remained at that of boiling 

 water, every cubic inch of steam above the water 

 in the flask would weigh about |th of a grain, since 

 100 cubic inches weigh about 15 gra : ns. Suppose the 

 capacity of the flask, exclusively of the fluid water 

 in it, to be 100 cubic inches. Then, to begin with, 

 the gaseous water which it contains will weigh 15 

 grains. If the flask is now allowed to cool, more 

 and more of the gaseous water condenses into the 



