236 Dr. Priestley’s Obfervations cn 
effeft might equally be attributed to this circumfiance... 
But he acknowledges, that this hypothecs ought not to 
be received till it be confirmed by experiments, MiJ'c .. 
\ Taurin . vol. V. p. 61. 
Upon the whole, he concludes, that the principal ufe of 
air to the blood, is to prelerve the equilibrium with the ex- 
ternal air, and to prevent the veflels from being rendered., 
unfit to tranfmit the blood, on account of the external, 
preffure; whereas, by means of the air they contain, the 
fluids move in their proper veflels as freely as in vacuo, 
and the membranes and vifcera alfo eafily hide over each 
other, p. 63. And with refpefl to the ufe of the lungs , 
lince he imagined that air is not introduced into the blood 
by means of them, he thinks, that becaufe fnch lungs as 
thofe of man are given to the warmer animals only, the 
chief ufe of refpiration is exhalation, and confequently 
the cooling of the blood, p. 65. 
The laft writer whom I fhall quote upon this fubjedf, 
is the late ingenious Mr. hewson; who fays, in his Ex- 
perimental Inquiry into the Properties of Blood , p. 9.. 
u As the colour of the blood is changed by air out of the 
u body, it is prefumed, that the air in the lungs is the 
“ immediate caufe of the fame change in the body.” 
That this change is really produced in the lungs, he is 
perfuaded, he fays, from experiments, in which he dif- 
tindfly faw the blood of a more florid red in the left au- 
ricle of the heart than it was in the right ; but how this 
effect is produced, he fays, is not yet determined. 
Since. 
