234 Dr. Priestley's Obfervations on 
noxious on the fame account as mephitic vapours, or tliofe 
of burning brim Hone, p. 3 1 ; that, in frequently breath- 
ing the fame air, it becomes fo loaded with thefe vapours, 
as to excite a convullion in the lungs, and thereby render 
them unfit for tranfmitting the blood, p. 42. 
This philofopher fuppofes that air enters the pores of 
the blood, retaining its elaftic power, p. 50. and that it 
continues at reft there, becaufe its endeavour to efcape is 
counteracted by the equal preflure of the ambient me- 
dium, p. 5 2. This air, he fuppofes to be introduced into 
the blood by the chyle, and never by the way of the 
lungs, except when, by fome means or other, the equili- 
brium between the air in the blood and the external air 
is loft, p. 57. If the external air be rarer than the inter- 
nal, the air in the blood, expanding itfelf, will inflate the 
animal, and have the fame eftecft as air introduced into 
the veins. 
What we are chiefly indebted to M. cigna for, is his 
decifive experiments with refpeCt to the florid colour of 
the blood, which he clearly proves to be caufed by the 
contaCt of air; though he afterwards feems willing to de- 
fert that hypothefts. It was often imagined, that the 
reafon why the lower part of a quantity of blood was 
black, while the furface w'as red, w T as, that the black par- 
ticles, being heavier than the reft, lubfided to the bot- 
tom; but this opinion our author clearly refutes. He 
found, that when he put a little oil upon a quantity of 
blood, it remained black throughout ; but that when he 
took away the red part, and expofed to the air the lower 
lamina. 
