208 Original Articles. [April, 



spectrum at 12, the centre of the upper band is at i or '4 ; of the 

 lower, If ; and of the green between them 1. The upper band has 

 a sharper outline, and, when so much blood is present as to cut off 

 about i of the spectrum, its width is about -f that of the lower, as 

 shown by No. 2. As a good example of an entirely different spectrum, 

 given by a solution of very similar colour, I refer to No. 1, which 

 represents that of a dilute solution of perchloride of iron to which a 

 little sulphocyanide of potassium has been added. In its case the 

 blue end of the spectrum is more and more absorbed, according to the 

 strength of the solution, but no narrow, dark bands are formed in any 

 part. The spectra of many red colouring-matters have this character, 

 and therefore could not possibly be confounded with blood. Cochi- 

 neal does indeed yield an absorption band in the same position as the 

 lower in fresh blood, but it is extremely faint, and there is none where 

 the upper band occurs. A second does exist, but it is 3£ below D, 

 and is seen only when the solution is so dilute that the blue part of 

 the spectrum is freely transmitted. On adding ammonia to the solu- 

 tion of cochineal, two absorption bands are produced, in such a posi- 

 tion that without care they might be confounded with those of blood ; 

 but the upper is decidedly broader than the lower, whilst it is the 

 reverse in blood ; and its centre is also lower, being ^ and not ^ from 

 D. A solution of cudbear in dilute alcohol also yields two very faint 

 bands in much the same place as those in fresh blood, but on adding 

 ammonia the band at i becomes very dark and distinct, and the lower 

 band vanishes. The addition of ammonia to a solution of fresh blood 

 produces no such changes. 



The gravy of roasted meat gives different spectra according to cir- 

 cumstances. The red liquid, which comes from it when under- 

 cooked, is merely a solution of cruorine, and gives the same spectrum 

 as fresh blood ; but the usual light or darker brown liquid sometimes 

 gives a dark and sharply-defined absorption -band at about 1*4 below 

 D, and suddenly cuts off all the rays below 2 J. The addition of am- 

 monia causes a precipitate, but leaves an obscure band in the same 

 situation. Citric acid removes the band entirely, but ammonia in 

 excess restores it without causing a precipitate ; and the addition of 

 protosulphate of iron produces no change. When dried and strongly 

 heated, the colour becomes darker, and it merely cuts off more or less 

 of the blue end of the spectrum, as in No. 1, without there being any 

 detached absorption-bands. Probably this is why dark-coloured gravy 

 often has that character. Thus, though, as far as tests are concerned, 

 the chemical composition is almost identical with that of blood, the 

 optical characters and the manner in which gravy behaves with re- 

 agents, suffice to distinguish it ; unless it be from meat so extremely 

 underdone, that it is in fact merely more or less modified blood. 



If blood be dried on white linen, and exposed to the air, the bright 

 scarlet colour changes by degrees into brown. This fact must be 

 well known, but so far as I am aware, has not been studied optically 

 or chemically. When the change has not proceeded far, the blood 

 yields a spectrum like that shown by No. 3. The two well-marked 

 bands in the green are much the same as in fresh blood ; but there is 



