Rcfpiration , and the Ufe of the Blood. 2 37 
Since fome of the neutral falts, and particularly 
nine, lias a fimilar effect on the colour of the blood; 
fome, fays lie, attribute this difference to the nitre ab- 
forbed from the air, while in the lungs. But this, he 
adds, is a mere hypotheffs, for air contains no nitre, 
and moil of the neutral falts produce the lame effect in 
fome degree. 
Since, however, a lblution of nitre does produce this 
effect upon blood, inftantly making the very blackeft of 
it of a beautiful florid red, though this effect is not pe- 
culiai to nitre (for a folution of common fait does nearly 
the fame thing) I own I am inclined to aferibe this effect 
to the air ; efpecially fince I have proved, as I apprehend, 
that atmofpherical air confifts of earth and fpirit of nitre. 
Poffibly, therefore, the air we breathe may be fo far de- 
compofed, as to communicate fomething of nitre to the 
blood, in its paflage through the lungs. 
After this review of the obfervations and opinions of 
others on this important queftion in phyfiology, I fhall 
proceed to recite my own. It may appear fomething ex- 
ti aordinary,that among fuch a variety of opinions concerti- 
ng the ufe of refpiration, the right one fhould never have 
been fo much as conjectured, though unfupported by the 
proper proof. But indeed, this animal function, and the 
phlogiftic proceffes in chemiftry, efpecially that of the 
calcination of metals, which is, perhaps, the molt fimple 
of them, are to appearance very different things ; and 
therefore, it is the lefs to be wondered, that no perfon 
fhould . 
