SECT. XXXVIII. 2. i. OF BLOOD. 367 



fides of the veflels. But no perceptible heat has ever been pro- 

 duced by the agitation of water, or oil, or quickfilver, or other 

 fluids ; except thofe fluids have undergone at the fame time fome 

 chemical change, as in agitating milk or wine, till they become 

 four. 



Befides the fuppofed production of phofphoric acid, and 

 change of colour of the blood, and the production of carbonic 

 acid, there would appear to be fomething of a more fubtile na- 

 ture perpetually acquired from the atmofphere; which is too 

 fine to be long contained in animal veflels, and therefore re- 

 quires perpetual renovation ; and without which perfect life 

 cannot continue longer than a minute or two ; this ethereal flu- 

 id is probably fecreted from the blood by the brain, and perpetu- 

 ally diflipated in the actions of the mufcles and organs of fenfe, 

 but which neverthelefs may remain for a longer time, where 

 there is little or no exertion of the animated fibres, as in fyncope, 

 and in thofe in feels and other animals, which remain during the 

 winter in a torpid (late, and may not entirely evaporate from, 

 defect of warmth, or moifture, or other circumftances, as fnails 

 are faid to have revived after having been many years in a dry 

 cabinet, and flies after having been many months drowned in 

 wine, and other infects after having been frozen. 



That the blood acquires fomething from the air, which is im- 

 mediately neceflary to life, appears from an experiment of Dr. > 

 Hare, (Philof. Tranfacl. abridged, Vol. III. p. 239.) who 

 found, " that birds, mice, &c would live as long again in a 

 veflel, where he had crowded in double the quantity of air by a 

 condenfing engine, than they did when confined in air of the 

 common denfity." Whereas if fome kind of deleterious vapour 

 only was exhaled from the blood in refpiration ; the air, when 

 condenfed into half its compafs, couid not be fuppofed to re- 

 ceive fo much of it. 



II. Sir Edward Hulfe, a phyfician of reputation at the begin- 

 ning of the prefent century, was of opinion, that the placenta 

 was a refpiratory organ, like the gills of fifh ; and not an organ 

 to fupply nutriment to the foetus ; as mentioned in Derham's 

 Phyfico-theology. Many other phyficians feem to have efpoufed 

 the fame opinion, as noticed by Haller. Elem. Phyfiologix, T. 

 I. Dr. Gipfon publifhed a defence of this theory in the Medi- 

 cal Eflays of Edinburgh, Vol. I. and II. which doctrine is 

 there controverted at large by the late Alexander Monro j and 

 fince that time the general opinion has been, that the placenta 

 is an organ of nutrition only, owing perhaps rather to the au- 

 thority of fo great a name, than to the validity of the arguments 

 adduced in its fupport. The fubject has lately been refumed by 

 ^ Dr. 



