248 Dr. Priestley’s Obfervations on 
in which it pofTefTes this power is eafily afcertained by 
the eye, it might not, perhaps, be unworthy of being par- 
ticularly attended to by phylicians. To eftimate the 
goodnefs of blood, according to this criterion, nothing is 
requifite but to obferve the lightnefs of the colour, and 
the depth of the light-coloured Jlratum , after it has heen 
expofed to the air for a given time. In cafes in which 
the blood is unufually black, and but little affected by 
common air, it fhould leem, that breathing a purer air 
might be prefer ibed with advantage. 
In general, the blood that I have been able to procure 
in the city has not been fo good as that which I have got 
in the country ; owing, perhaps, to the cattle having been 
much driven, and heated before they were killed. 
XIV. An 
