150 



ON THE CIRCULATION OF THE BLOOD, 



parts is produced in the same manner. Whether the iron and sulphuir 

 that are traced in the blood have a similar origin, or exist in the different 

 articles of our diet, and are merely separated from the other materials with 

 which they are combined, is a physical problem that yet remains to be 

 solved. It should be observed, however, that the sulphur does not exist 

 in a free state even in the blood itself, but is only a component part of its 

 albumen. Considering the universality of these substances in the blood, 

 and the uniformity of their proportion in similar ages, temperaments, and 

 habits, whatever be the soil on which we reside ; that those who hve in a 

 country in which these minerals are scarcely to be traced have not less, 

 while those who live in a country that overflov/s with them have not more j 

 it is perhaps most rational to conclude, that they are generated in the 

 laboratory of the animal system itself, by the all-controUing influence of 

 the living principle. 



The exact proportion of sulphur contained in the system has been less 

 accurately ascertained than that of the iron, which last in an adult, the 

 weight of whose blood may be estimated at 28lbs.,* ought usually to 

 amount to seventy scruples, or about three ounces : and hence the blood 

 of about forty men contains iron enough to make a good plough-share, 

 and might easily have its iron extracted from it, be reduced to a metallic 

 state, and manufactured into such an instrurflent. 



Iron is seldom found except in the red particles of the blood ;t and it 

 has hence been supposed by the French chemists to be the colouring ma- 

 terial itself. The process of respiration, according to the theory of La^ 

 voisier and Fourcroy, is a direct process of combustion in which the ani- 

 mal system finds the carbone, and the atmosphere the oxygene and caloric ; 

 and in consequence of the sensible heat which is set at liberty during the 

 combustion, the iron of the blood is converted into a red oxyde, and hence 

 necessarily becomes a pigment. 



But it is impossible to ascribe the red colour to this principle : for, first, 

 we are by no means certain that the air communicates any such substance 

 as caloric to the blood ; and, secondly, let the sensible heat of the blood ' 

 arise firom whatever quarter it may, it can never be sufficiently augmented 

 by the most violent degree, either of local or general inflammation, to con- 

 vert the iron of the blood into a red oxyde, which, indeed, is never pro- ' 

 duced without rapid combustion, flame, and intense heat. And hence. Sir 

 Humphry Davy conjectures the carbon itself of the blood to be the real 

 colouring material, and to be separated from the oxygene, with which it is 

 necessarily united to constitute carbonic acid gas, by the matter of light, 

 which he supposes to be introduced into the system in the act of respira- 

 tion, instead of the matter of caloric ; in consequence of which it immedi- 

 ately becomes a pigment. But the difficulties which attend this theory 

 are almost, if not altogether, as numerous as those which attend the theory 

 of combustion, and it is unnecessary to pursue th^e subject any farther. 

 In the Philosophical Transactions, and in several of the best established 



* Blumenbach states the proportion in an ^dult and healthy man tcbe as 1 to 5 of the entire 

 weight of the body. By experiments on the water-newt (lacerta jjo/ttifm), he found the 

 proportion in this animal to be only as 2^ to 36. 



t Mr. Brandc denies that iron exists more in the red particles of the blood than in the other 

 principles : according to his experiments, it exists but in a very inconsiderable quantity in 

 any of them ; but he has traced it in the chyle, in the serum, and in the fibrine, or washed 

 crassament. Phil. Trans 1812, p. 112. Vauquelin has traced it as a constituent in esg- 

 shells and oyster-shelk. Thomson's Annals of Philos. No. 1. p. 66. But Berzelius has 

 proved Brande to be mistaken, and that iron exists larg;ely in the blood, and is the cause of 

 !he red colour. See his Anira. Chemistrv. 



