78 ELEMENTS OF SCIENCE 



the greatest ease, and a liquid presses equally in all 

 directions. In aeriform bodies, however, not only do 

 the particles not cohere, but they actually repel each 

 other and separate as far as possible, pressing, however, 

 equally in all directions. This tendency of an aeriform 

 body to spread and diffuse itself, is spoken of as an 

 " extreme elasticity," and it is accompanied by an 

 extreme degree of compressibility. Both these extremes 

 are characteristic of aeriform bodies exclusively. Never- 

 theless, like solids and liquids, they possess weight, inertia, 

 momentum, and impenetrability^not, of course, that a 

 mass of air is impenetrable, and we may, with a finger, 

 penetrate even into the mass of a soft solid ! But the 

 real essential substance of aeriform, as of all other, 

 matter, is deemed to be impenetrable in the sense that 

 it must always remain of some dimension and cannot be 

 made actually nothing of. 



Aristotle was aware that air was a material substance 

 and, like other bodies possessing weight, tended to 

 descend towards the earth. In fact its weight is very 

 considerable, and greatly modifies the circumstances and 

 actions of liquids, so that some additional facts about 

 the latter will have to be noted before concluding this 

 chapter. Such is the case because, in treating of liquid 

 bodies, we made abstraction of the action of aeriform 

 ones as being things which had not yet been brought 

 before the reader's cognisance. 



The weight of the atmosphere i.e., of the aerial mass 

 round the surface of the earth, at the sea-level is 

 between fourteen and fifteen pounds upon every square 

 inch. We say " at the sea-level," because it is obvious 

 that the more we ascend above this level the less will be 

 the volume of air which presses downwards. But the 

 decrease in pressure is very far from being uniform, 



