74 Experimental Philosophy. [Lecture 6, 



sure of air upon it, and the fluids of the body are 

 driven to that part where there is least resistance. 



By the air-pump we are also convinced more 

 clearly of the elasticity and compressibility of 

 the air. Take a bladder from which the air is 

 almost totally exhausted, and which appears 

 (juite flaccid and compressed, tie the neck of it 

 tight as it was when full, and put it in an air- 

 pump. As the air is exhausted we shall see the 

 bladder gradually inflate, till, at length, it will be 

 puffed out to the full size it was before we had 

 expelled the air. Mr. Boyle relates that, by 

 means of the air-pump, he had rarefied common 

 air so as to make it fill nearly fourteen thousand 

 times the space it did before. 



A similar effect would take place with a blad- 

 der, by carrying it to the higher regions of the 

 atmosphere, where, as before explained, the air 

 is thinner and lighter, and consequently its pres- 

 sure less. If a bladder half full is carried up to 

 the top of a high mountain, it will gradually di- 

 late to its former size. 



If, instead of a bladder almost empty, a full- 

 blown bladder, or a thin glass bubble filled with 

 air. and closely stopped, is put into the ah -pump, 

 as soon as the air is exhausted, the bladder or the 

 bubble will burst in pieces. 



The air is also capable of being rarefied by 

 heat. If a bladder, half blown and tightly tied 

 at the neck, is held to the fire, we shall find that 

 it will dilate to nearly its full size ; and if either a 



