on the Colour of Blood. 427 
I proceed next to offer a few observations upon 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 argu- 
ment 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 va- 
rious parts of animals without colour, 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 
permanently 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 cqlour 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 upon 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. 
3 K 
MDCCXCVII. 
