IHE EARTH. 205 



put firmly down, woulii 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 extraordinary, 

 (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 elas- 

 ticity of the air contained within, struggling to escape, and 

 breaking in pieces all the substances with which it is mixed. 

 Care, however, must be taken not to heat this instrument too 

 violently : for then the inclosed air would become irresistible, 

 and burst the whole, with, perhaps, a fatal explosion. 



There are numberless other useful instruments made to de- 

 pend on the weight, the elasticity, or the fluidity, of the air, 

 which do not come within the plan of the present work ; the 

 design of which is not to give an account of the inventions that 

 have been made for determining the nature and properties of 

 air, but a mere narrative of its effects. The description of the 

 pump, the forcing-pump, the fire-engine, the steam-engine, the 

 syphon, and many others, belong not to the naturalist, but the 

 experimental philosopher : the one gives a history of Nature, as 

 he finds she presents herself to him ; and he draws the obvious 

 picture: the other pursues her with close investigation, tortures her 

 by experiment to give up her secrets, and measures her latent 

 qualities with laborious precision. Much more, therefore, might 

 be said of the mechanical eflfects ot air, and of the conjectures 

 that have been made respecting the form of its parts ; how some 

 have supposed them to resemble little hoops, coiled up in a 

 spring ; others, like fleeces of wool ; others, that the parts are 

 endued with a repulsive quality, by which, when squeezed toge- 

 ther, they endeavour to fly oflT, and recede from each other. We 

 might have given the disputes relative to the height to which 

 this body of air extends above us, anjj concerning which there js 

 no agreement. "We might have inquired how much of the air 

 we breathe is elementary, and net reducible to any other sub- 

 stance ; and of what density it would become, if it were sup- 

 posed to be continued down to the centre of the earth. At that 

 place we might, with the help of figures, and a bold imagination, 

 have shown if twenty thousand times heavier than its bulk of 



s 



