PNEUMATICS. 1§7 



its weight, bulk for bulk, the higher it rises ; 

 4. It cannot be congealed or fixed as other fluids 

 may. 



Few people who are unacquainted with the 

 principles of natural philosophy, suppose that the 

 air, by which we are surrounded, is a material 

 substance, like water, or any other visible matter. 

 Being perfectly invisible, and affording no resist- 

 ance to the touch, it must seem to them extraor- 

 dinary, to consider it as a material substance ; 

 and yet a few simple experiments will convince 

 any one that it is really matter, possessing weight, 

 and the power of resisting other bodies that press 

 against it. 



Take a bladder that has not the neck tied, and 

 you may press the sides together, and squeeze it 

 into any shape. Fill this bladder with air by 

 blowing into it, and tie a string fast round the 

 neck : you then find that you cannot, without 

 breaking the bladder, press the sides together, and 

 that you can scarcely alter its figure by any pres- 

 sure. Whence then arise those effects ? when the 

 bladder was empty, you could press it into any 

 form ; but the air with which it is rilled prevents 

 this: the resistance you experience, when it is filled 

 with air, proves that that is as much matter as any 

 other substance that we are acquainted with. 



We are accustomed to say, that a vessel is empty, 

 when we have poured out of it the water which it 

 contained. Throw a bit of cork upon a bason of 

 water, and having put an empty tumbler over it, 

 with the mouth downwards, force it down through 

 the water ; the cork will show the surface of the 

 water within the tumbler, and you will see that it 

 will not rise so high within as without the glass ; 

 nor, if you press ever so hard, will it rise to the 



