I04 THEPHILOSOPHY 



that purpofe, forces out the air, and thus removes the oiFending 

 caufe. The lungs, after the air is thrown out, inftead of being in- 

 flated, collapfe ; and, if a frefh fupply is not foon taken in, a fimilar 

 nneafy fenfation is felt, which obliges the animal again to infplre. 

 This alternate reception and rejection of air goes on during the life 

 of the animal, and is diftinguifhed by the general name of refpira' 

 tion. But, when treating more accurately of the fubjeft, the a<3: of 

 taking air into the lungs is called hi/piration, and the ad of throw- 

 ijig it out is termed expiration. 



That the refpiration of air is indifpenfible to the exiftence of land- 

 animals, has been proved by innumerable experiments made with 

 the air-pump. Mice, rats, rabbits, cats, dogs, &c. when placed in 

 an cxhaufted receiver, inftantly become refllefs, and difcover fymp- 

 toms of pain. Their bodies fwell, and their life is foon extinguifh- 

 €d. Indeed, our own feelings are fufEcient to afcertain this fad. 

 No perfon can remain long either in a ftate of infpiration or expira- 

 tion without being fuffocated. 



But the alternate motions of infpiration and expiration, joined to 

 the circulation of the blood through the lungs, may be confidered as 

 the more mechanical effeds of refpiration. Though thefe opera- 

 tions are abfolutely necefTary to the exiftence of animals, yet the air 

 itfelf has been fuppofed to impart fome vital principle to the blood, 

 without which life could not be continued. 



The ingenious Dodor 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 men- 

 tioning a well known fad, that all bodies, whether animate or in- 

 animate, contain a certain quantity of fire as a principle in their 

 compofition, the Dodor remarks, that this quantity, in different bo- 

 dies, 



