52 SCIENCE PRIMERS. [MATERIAL 



the weight of the steam would be exactly the same 

 as that of the water. If you had a small square cup 

 like a die, the inside measure of which was exactly 

 one inch each way, it would hold one cubic inch of 

 water. If this cup full of water were heated till all 

 the water was turned into steam, the steam would 

 nearly occupy a cubic foot, since there are 1,728 

 cubic inches in a cubic foot. A cubic inch of water 

 weighs 25 2 J grains, and the steam into which it is 

 converted has just the same weight. Thus we may 

 say that steam is water expanded by heat into a 

 vapour which is of 1,700 times less specific gravity than 

 water. On the other hand, a pint of steam allowed 

 to cool, becomes converted into a quantity of water, 

 which measures only t -^th of a pint, though it weighs 

 just as much as the whole pint of steam did. The 

 steam, therefore, is condensed to a yyV^h of its 

 volume of water. 



The power with which water expands when it is 

 converted into steam is very great. If you were to 

 stop up the nozzle of the tea-kettle, the steam, inside 

 the kettle, in trying to expand, would burst open the 

 lid ; and if you were to fasten down the lid, it would 

 pretty soon burst the kettle itself. You sometimes 

 hear of the strong boilers of steam-engines being 

 burst in this way. 



35. Gases or Elastic Fluids. Air. 



Here is a glass flask with a long neck and an open 

 mouth. If we pour water in at the mouth until it 

 rises to the lip we say that the flask is full of water. 

 If we now pour the water out we say that the flask is 

 empty. But is it empty? Press the flask mouth 



