THE EARTH, 



The air-pump is an instrument contrived to 

 exhaust the air from round a vessel adapted to 

 that purpose, called a receiver. This method of 

 exhausting is contrived in the simple instrument, 

 by a piston, like that of a syringe, going down 

 into the vessel, and thus pushing out its air ; 

 which, by means of a valve, is prevented from 

 returning into the vessel again. But this, like 

 all other complicated instruments, will be better 

 understood by a minute's inspection, than an 

 hour's description ; it may suffice here to observe, 

 that by depriving animals, and other substances, 

 of all air, it shows us what the benefits and effects 

 of air are in sustaining life, or promoting vege- 

 tation. 



The digester is an instrument of still more exr 

 traordinary effects than any of the former ^ and 

 sufficiently discovers the amazing force of air, 

 when its elasticity is augmented by fire. A com- 

 mon tea-kettle, if the spout were closed up, and 

 the lid put firmly down, would serve to become 

 a digester, if strong enough. But the instrument 

 used for this purpose is a strong metal pot, with 

 a lid to screw close on, so that, when down, no 

 air can get in or return ; into this pot^ meat and 

 bones are put, with a small quantity of water, 

 and then the lid screwed close ; a lighted lamp 

 is put underneath, and what is very extraordi- 

 nary, yet equally true, in six or eight minutes 

 the whole mass, bones and all, are dissolved into 

 a jelly ; so great is the force and elasticity of the 

 air contained within, struggling to escape, and 

 breaking in pieces all the substances with which 



