THE BLOOD. 11 



is blackish (v), and that which comes afterwards is more florid : 

 in such cases, the arterial blood passes into the veins without 

 undergoing that change which is natural to it. 



Some of the neutral salts have a similar effect on the colour 

 of the blood to what air has, particularly nitre ; thence some 

 have attributed the difference of colour in the arterial and 

 venous blood to nitre, which they supposed was absorbed from 

 the air whilst in the lungs. But we know that this is a mere 

 hypothesis, for air contains no nitre (vi). Indeed, nitre is far 

 from being the only neutral salt which has this effect on the 

 blood, for most of them have some degree of it. In making 

 some experiments on this subject, I have observed a more re- 

 markable effect which some of the neutral salts have upon the 

 blood ; and that is, being mixed with it when just received 

 from the vein, they prevent its coagulation, or keep it fluid, 

 and yet, upon adding water to the mixture, it then jellies or 

 coagulates. Thus, if six ounces of human blood be received 

 from a vein upon half an ounce of true Glauber's salt reduced 

 to a powder, and the mixture agitated so as to make the salt 

 be dissolved, that blood will not coagulate on being exposed 



(v.) Blood quickly becomes darker by stagnation in the living body. 

 In bleeding a horse, if the neck be tied up a few minutes, the first 

 flowing blood from the vein will be very dark coloured. Two hours 

 after putting two ligatures on the femoral artery of a dog, I found the 

 included blood as dark as that of a vein. Mr. Hunter a noticed a 

 darkening of the arterial blood merely from the stagnation produced by 

 a tourniquet. At the temperature of the body changes take place very 

 rapidly in the blood. Its darkening from stagnation may perhaps be 

 owing to the conversion of a little oxygen, in contact with the red cor- 

 puscles, into carbonic acid. b 



(YI.) Perhaps Mayow's nitro-aerial spirit, mentioned in Note iv, 

 may be here confounded with nitre. But when Hewson wrote, there 

 was an opinion current that the air in many places is impregnated with 

 nitre, which he correctly opposes as a mere hypothesis. The know- 

 ledge then possessed of the atmosphere was very vague. Oxygen, Dr. 

 Priestley's dephlogisticated air, was discovered by him in August 1/74, 

 three years after the publication of this part of Hewson's works. 

 Among the earlier observers on the effects of salts on the colour and 

 coagulation of the blood, were Dr. Freind, c J. Handley, d and Thomas 

 Schwenke. 6 



a On the Blood, Palmers ed. p. 88. d Mec. Essays on the Animal Economy, 

 b See Dr. Davy's Researches, ii, 210. p. 8, 8vo, Lond. 1721. 



c Emmenologia, p. 160, 1, 12, 13, 14, e Haematologia, pp. 106, 145 etseq. 8vo, 

 8vo s Oxon. 1703. Hagae Com. 1743. 



