Spectrum MetJiod of Detecting Blood. 15 



double tartrate and ammonia, and remains so when deoxidized. 

 The hand of deoxidized haematin can however he distinctly seen with 

 a light sufficiently strong to penetrate the turhid and dark solution. 

 Before examining the suspected stain, it would he well to make out 

 how much of the unstained leather could be used without giving 

 too dark a solution, and to use no more of the stained. If the deoxi- 

 dized solution be too turbid, the cell may be kept for a while hori- 

 zontal, until the deposit has subsided sufficiently to allow the 

 principal absorption-hand to be seen ; but it is not so distinct, when 

 all has subsided, as though the greater part of the htematin still 

 existed as a compound insoluble in dilute ammonia. 



The presence of tannic acid in wood and other substances might 

 make it necessary to employ a similar process, if the relative amount 

 of blood were so small, that none could be dissolved out by water, 

 or dilute citric acid. 



Cases might occur when it would be necessary to decide whether 

 blood were present, along with some other coloured substance, 

 soluble in water. The method to be employed would depend much 

 on the nature of this impurity. If it were a coloming matter, 

 belonging to what I have described in former papers as group A, 

 in which the absorption is removed by sulphite of soda, in an 

 alkaline solution, there would be no difficulty in seeing all the 

 spectra. Thus, for example, it is easy to add so much magenta to 

 the solution of a little blood, that its absorption-bands are entirely 

 hid ; but a small quantity of sulphite of soda so completely removes 

 the colour of the magenta, that the various spectra of the blood 

 may be seen almost as well as if it had been pure. 



The coloming matters of my group B that are most likely to 

 occur, are those of fruits, and in them the presence of the free acid 

 would be almost certain to have changed the haemoglobin into 

 hsematin. The best plan would then be to add excess of ammonia, 

 and, if the solution were made too dark, to dilute it with so much 

 water that the strongest light at our command would show the 

 green part of the spectrum sufficiently bright to prove that no 

 absorption-band occurred there. On deoxidizing in the usual 

 manner, the solution may be made somewhat darker by the presence 

 of tannic acid, but the darker band of deoxidized haematin could 

 be recognized without material difficulty. 



By far the greater number of the colouring matters belonging 

 to my group C are yellow and orange-coloured ; and, since these 

 chiefly absorb the blue rays, they do not interfere with our seeing 

 the bands of the blood spectra, which occur in the green. Cochineal 

 is one that requires special attention. The addition of ammonia to 

 its solution in water gives rise to two bands in the green, which, 

 though differing materially from those of blood, are yet so nearly 

 in the same situation, that they completely disguise the presence of 



VOL. VI. c 



