THE BLOOD. 149 



If arterial blood is exposed to ammoniacal gas, it becomes of 

 a cherry red ; if to gaseous oxide of carbon, the deutoxide of 

 azote, or carburetted hydrogen, of a violet red ; if to carbonic 

 acid, though covered by two inches depth of serum 8 , azote, hy- 

 drogen, or protoxide of azote, of a brown red ; if to arseniuretted 

 or sulphuretted hydrogen, of a deep violet inclining to a greenish 

 brown ; if to hydrochloric gas, of a chestnut brown; if to sulphu- 

 reous gas, of a blackish brown; and if to chlorine, of a blackish 

 brown inclining to a greenish white. 4 These gases are of course 

 partly absorbed or decomposed. The dark colour produced in 

 arterial blood by carbonic acid or azotic gas takes place if blood 

 is placed in vacuo u , though less rapidly and deeply than if ex- 

 posed to hydrogen gas, and in vacuo, though covered by two 

 inches and a half of serum. x Arterial blood extravasated or in- 

 cluded between two ligatures in an artery y , nay, left in contact with 

 oxygen, gradually acquires the same dark colour, and no oxygen 

 will afterwards render it scarlet. Acids, if stronger than just 

 enough to neutralise a weak salt, and alcalies, darken arterial 

 blood. If deprived of all saline matter by washing, the reddest 

 clot of blood becomes black, and will not grow florid by oxygen: 

 whereas if saline matter is added, it becomes florid, even in an 

 atmosphere of carbonic acid. 2 



Venous blood acquires a florid colour by exposure to oxygen 

 or atmospheric air (and it does so even when covered by a blad- 

 der, provided this is moistened a ), carbonic acid gas is formed, 

 and an equal volume of oxygen gas disappears, and this the 

 more if the temperature is high. b If exposed to nitrous oxide, it 

 becomes of a brighter purple, and much of the gas is absorbed ; 

 carbonic acid gas renders it darker, and is a little absorbed : 

 nitrogen and hydrogen have the same effect. Electricity blackens 

 the blood, and, according to Dr. Stevens, the poison of the rattle- 

 snake, and other poisons, though floating only in the air. Putre- 

 faction makes the blood dark. Alcaline and some other neutral 



s Dn Priestley, Ph. Tr. 1776* 

 < Raspail, 1. c. p. 361. 



u Beccaria, who experimented at the request of Cigna. Misc. Taur. t. i. 

 x Dr. Priestley. 



y Hunter, On the Stood, p. 65. sq. 

 z Dr. Stevens. 



a A layer of serum or milk does not prevent this change of colour, while a 

 layer of water, saliva, and every other animal fluid, or oil, does. Dr. Priestley. 

 b Dr. Stevens. 



