OF NATURAL HISTORY. lig 



tingulfhed by the general name of refpiratioju But, whea 

 treating more accurately of the fubje^l, the a6l of taking air 

 into the lungs is called infpirationy and the a<St of throwing it 

 out is termed expiration. 



That the refpiration of air is indifpenfible to the exiftence 

 of land animals, has been proved by innumerable experi- 

 ments made with the air-pump. Mice, rats, rabbits, cats, 

 dogs, &c. when placed in an exhaufted receiver, inftantly 

 become reftlefs, and difcover fymptoms of pain. Their 

 bodies fwell, and their life is foon extinguifhed. Indeed, 

 our own feelings are fufficient to afcertai^i this fadl. No 

 perfon can remain long either in a fiiate of infpiration or ex- 

 piration without being fuffocated. 



But the alternate motions of infpiration and expiration, 

 joined to the circulation of the blood through the lungs, may 

 be conlidered as the more mechanical effe^^s of refpiration. 

 Though thefe operations are abfolutely necefTary to the ex- 

 iftence of animals, yet the air itfelf has been fuppofed to im- 

 part fome vital principle to the blood, without which life 

 could not be continued. 



The ingenious Dodlor Crawford, in his treatife on Animal 

 Heat, has rendered it probable, that the refpiration of air is 

 the caufe of that vital warmth without which no animal can 

 exift. After mentioning a well known fadl, that all bodies, 

 whether animate or inanimate, contain a certain cv^antity of 

 fire as a principle in their compofition, the Do^lor remarks, 

 that this quantity, in different bodies, varies according to 

 their nature or texture ; that this fire, when in a latent or 

 quiefcent ftate, is termed ahfolute heat \ that, when fubflances 

 of different textures have a given quantity of heat thrown in- 

 to them, their temperature will be difcovered to be difl'erent 

 by the thermometer j for the fame quantity of heat which 

 raifes one body to a certain degree, will raife another to a 



