392 Royal Society : — 



alkaline carbonates and caustic ammonia, but is almost immediately 

 decomposed by acids, and also, but more slowly, by caustic fixed 

 alkalies, tbe coloured product of decomposition being tbe hsematine 

 of Lecanu, which is easily identified by its peculiar spectra. But 

 it seemed to me to be a point of special interest to inquire whether 

 we could imitate the change of colour of arterial into that of venous 

 blood, on the supposition that it arises from reduction. 



2. In my experiments I generally employed the blood of sheep or 

 oxen obtained from a butcher ; but Hoppe has shown that the blood 

 of animals in general exhibits just the same bands. To obtain the 

 colouring matter in true solution, and at the same time to get rid of 

 a part of the associated matters, I generally allowed the blood to 

 coagulate, cut the clot small, rinsed it well, and extracted it with 

 water. This, however, is not essential, and blood merely diluted 

 with a large quantity of water may be used ; but in what follows it 

 is to be understood that the watery extract is used unless the contrary 

 be stated. 



3. Since the colouring matter is changed by acids, we must employ 

 reducing agents which are compatible with an alkaline solution. If to 

 a solution of protosulphate of iron enough tartaric acid be added to 

 prevent precipitation by alkalies, and a small quantity of the solu- 

 tion, previously rendered alkaline by either ammonia or carbonate of 

 soda, be added to a solution of blood, the colour is almost instantly 

 changed to a much more purple red as seen in small thicknesses, and 

 a much darker red than before as seen in greater thickness. The 

 change of colour, which recalls the difference between arterial and 

 venous blood, is striking enough, but the change in the absorption 

 spectrum is far more decisive. The two highly characteristic dark 

 bands seen before are now replaced by a single band, somewhat 

 broader and less sharply defined at its edges than either of the 

 former, and occupying nearly the position of the bright band sepa- 

 rating the dark bands of the original solution. The fluid is more 

 transparent for the blue, and less so for the green than it was before. 

 If the thickness be increased till the whole of the spectrum more 

 refrangible than the red be on the point of disappearing, the last 

 part to remain is green, a little beyond the fixed line b, in the case 

 of the original solution, and blue, some way beyond F, in the case 

 of the modified fluid. Figs. 1 and 2 in the accompanying woodcut 

 represent the bands seen in these two solutions respectively. 



4. If the purple solution be exposed to the air in a shallow vessel, 

 it quickly returns to its original condition, showing the two charac- 

 teristic bands the same as before ; and this change takes place imme- 

 diately, provided a small quantity only of the reducing agent were 

 employed, when the solution is shaken up with air. If an additional 

 quantity of the reagent be now added, the same effect is produced as 

 at first, and the solution may thus be made to go through its changes 

 any number of times. 



5. The change produced by the action of the air (that is, of 

 course, by the absorption of oxygen) may be seen in an instructive 

 form on partly filling a test-tube with a solution of blood suitably 



