THE EARTH 125 



instruments, will be better understood by a minute inspection, than 

 an hour's description : it may suffice here to observe, that by depriving 

 animals, and other substances, of all air, it shews us what the benefits 

 and effects of air are in sustaining life, or promoting vegetation. 



The digester is an instrument of still more extraordinary effects than 

 any of the former ; and sufficiently discovers the amazing force of air, 

 when its elasticity is augmented by fire. A common tea-kettle, if the 

 spout were closed up, and the lid put firmly down, would serve to be- 

 come 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 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 depend 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 determin- 

 ing 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 natu- 

 ralist, 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, tor- 

 tures 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 effects of 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 together, they endeavour to fly off, and recede 

 from each other. We might have given the disputes relative to the 

 height to which this body of air extends above us, and concerning 

 which there is no agreement. We might have inquired how much of 

 the air we breathe is elementary, and not reducible to any other sub- 

 stance ; and of what density it would become, if it were supposed 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 it twenty 

 thousand times heavier than its bulk of gold. We mignt aiso prove it 

 millions of times purer than upon earth, when raised to the surface of 

 the atmosphere. But these speculations do not belong to natural his- 

 tory ; and they have hitherto produced no great advantages in tht 

 branch of science to which they more properly appertain. 



