196 



presence of this substance in the blood, the black 

 colour of that fluid has been attributed. Formerly, 

 indeed, this blackness of colour was ascribed to the 

 agency of phlogiston, or of hydrogen : but as the 

 existence of the former principle is now altogether 

 denied, and the latter substance can be shewn to re- 

 side in the blood only as a constituent part, as it does 

 in the other fluids and solids which are of various co- 

 lours, we must reject the opinion of its sufficiency 

 to account for the black colour of the blood. Be- 

 cause also red blood became black when placed in 

 nitrogen gas, by some, that gas has been held to be 

 the cause of its blackness ; but, according to Mr 

 Davy, no affinity subsists between nitrogen gas and 

 blood *. The same effect takes place also, when 

 blood is exposed to the contact of other gases, and 

 even in 'vacuo without the agency of any gas at all. 

 Dismissing then these opinions concerning the cause 

 of the black colour of the blood, let us next inquire 

 how far it may, with justice, be attributed to the 

 presence of carbon. 



155. That carbon is a constituent element of ani- 

 mal substances, is generally allowed ; and that it ex r 

 ists in the blood, and is yielded by it to form carbo- 

 nic acid, have likewise been admitted : but that, to 

 this carbon, the black colour of the blood is, or can 

 be, owing, has not hitherto been distinctly proved. 

 It remains to-be shewn in what state this carbon ex- 

 ists, and by what agency it can produce this colour ; 

 for in its pure state of diamond, nothing is farther 



Researches, p. 375. 



