ON THE COLOUR OF THE BLOOD. 219 



does not become coloured without the concourse of air, and that 

 as oxigene alone is absorbed in respiration, it appears that the co- 

 lour is owing to iron calcined by the pure air, and reduced to 

 the state of a red oxide." This evidently, is advancing one 

 step further towards the truth than the foregoing doctrines, ne- 

 vertheless he has stopped short of explaining the true pheno- 

 menon, for it is very manifest that he has endeavoured to prove 

 in all his experiments, that oxigene gas is nothing but oxigene 

 and. caloric. But, by the experiments of Mr. Berthollet, it ap- 

 pears, that "oxigene and light have great affinity, that light is 

 susceptible of combining with it, and that it contributes along 

 with caloric to change it into the state of gas." Mr. Fourcroy 

 in his general system of chemistry, and particularly on the co- 

 louring part of the blood, has offered the following theory. 



He says " it must be observed that there are two phosphates 

 of this metal," alluding to iron, " the one white-grey, fre- 

 quently of a pearly brilliancy, insoluble in water, soluble in 

 the acids ; and the other red, more or less brown, and less so- 

 luble in the acids; this is phosphate with excess of oxide of 

 iron, and the other is saturated with its acid." "The white 

 phosphate of iron is decomposed only in a partial manner by 

 the caustic alkalis, which take from it only a part of its acid, 

 and leave the salt with an excess of this base. It is in this state 

 of phosphate supersaturated with iron, a state maintained by 

 the presence of the soda, that this metal is dissolved in the 

 blood, and in particular in its serum. The blood of all ani- 

 mais, when it is red, is coloured by the phosphate of iron," 

 How far this theory corresponds with the laws of the affinities, 

 and the experiments of eminent chemists, the following ob- 

 servations will shew. If the phosphorus in the blood has a 

 greater attraction for the oxigene taken in during respiration 

 than the iron, the phosphoric acid must consequently first take 

 place, before the phosphate of iron can be produced. If we 

 take for granted that the phosphoric acid is produced in the 

 blood, the soda with which the blood abounds, having a greater 

 affinity to phosphoric acid, than what iron has, the phosphate 

 of soda must be the necessary result, and the iron would con- 

 sequently be left free. For the experiments of Mr. Lavoi- 



