54 SCIENCE PRIMERS. [MATERIAL 



the flask in the preceding experiment. The reason 

 of this is that the water compresses the air into a 

 smaller volume. A bag full of air, such as a common 

 air-cushion, can be squeezed till the air in its interior 

 occupies a much smaller volume; and, if you treat a 

 syringe full of air in the same way as the syringe full 

 of water was treated, you will find, if the piston fits 

 well, that it can be driven down some distance and 

 then springs back again. Air in fact is not only a 

 compressible, but it is an elastic fluid or gas. Heat 

 expands air just as it expands water, but the expansion 

 of air for the same degree of heat is much greater. 



36. Steam is an Elastic Fluid or Gas. 



In all the properties which have been mentioned 

 water in the form of steam is an elastic fluid or gas 

 like air. 



If a little water is placed in the flask mentioned 

 in the preceding section all the " empty " part of the 

 space will contain air. If the flask is now made hot 

 the water will at length boil, bubbles of steam forming 

 in the water and breaking at its surface. By degrees, 

 the air, which at first lay above the water, will be 

 driven out ; and, if the whole flask is kept hot, the 

 "empty" part of it will be full of the gaseous water, 

 which is transparent and colourless like air. The 

 steam flows out of the mouth of the flask still a clear 

 and colourless gas ; but it soon cools and becomes 

 condensed as a cloud of small particles of fluid water. 



Steam is lighter than air, and hence it rises in the 

 air; just as bodies which are lighter than water rise 

 in water. 



