234 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q7. 



and the mixture will appear brighter, if seen by reflected light. The last I shall 

 note is, that the red solution ought to be poured gently from the vessel in which it 

 has been made. If it be not, as it is a mucilaginous liquor, it is apt to entangle 

 small particles of air, which by acting as opake matter, will for some time alter the 

 appearance of the solution. 



I proceed next to offer a few observations on the cause of the red colour of blood. 

 It has of late been very generally supposed, that blood derives its colour from iron. 

 As far as I know however, no other argument has been given in support of this 

 opinion, than that the red matter is found to contain that metal. But there is 

 certainly no necessary connection between redness and iron; since this metal exists 

 in many bodies of other colours, and even in various parts of animals without co- 

 lour, as bones and wool. More direct reasons however may be given for rejecting 

 this opinion. 1 . I know of no colour, arising from a metal, which can be perma- 

 nently destroyed by exposing its subject, in a close vessel, to a heat less than that 

 of boiling water. But this happens with respect to the colour of blood. 2. If 

 the colour from a metal, in any substance, be destroyed by an alkali, it may be 

 restored by the immediate addition of an acid; and the like will happen from the 

 addition of a proper quantity of alkali, if the colour has been destroyed by an acid. 

 The colour of blood, on the contrary, when once destroyed, either by an acid or 

 an alkali, can never be brought back. 3. If iron be the cause of the red colour of 

 blood, it must exist there in a saline state, since the red matter is soluble in water. 

 The substances therefore, which detect almost the smallest quantity of iron in such 

 a state, ought likewise to demonstrate its presence in blood; but on adding Prussian 

 alkali, and an infusion of galls, to a very saturate solution of the red matter, I 

 could not observe, in the former case, the slightest blue precipitate, or in the latter, 

 that the mixture had acquired the least blue or purple tint. 



On the whole it appears to me, that blood derives its colour from the peculiar 

 organization of the animal matter of one of its parts; for whenever this is destroyed, 

 the colour disappears, and can never be made to return ; which would not I think 

 be the case, if it depended on the presence of any foreign substance whatever. I 

 shall conclude this paper with relating several miscellaneous facts respecting the co- 

 lour of blood, and some conclusions which may be formed from them. 



Dr. Priestley has mentioned, (Phil. Trans., 1776, p. 246) that the only animal 

 fluid, besides serum, which he found to transmit the influence of common air to 

 blood, was milk. But I have observed, that the white of an egg possesses the same 

 property, notwithstanding its great tenacity. Now as serum contains an animal 

 substance very similar to the white of eggs, it occurred to me as a question, whe- 

 ther, in transmitting the influence of air to blood, it acts by its salts only, or partly 

 by means of the substance of which I have just spoken. I took therefore a quan- 

 tity of urine, which is known to contain nearly the same salts as serum, and hav- 

 ing added to it as much distilled water as rendered its taste of the same pungency as 

 that of serum, I poured the mixture on a piece of dark crassamentum of blood. 



