212 Original Articles. [April, 



important investigations, it would be advisable to examine the spectra 

 of any substance likely to be present. I would also specially insist 

 on the importance of carefully attending to tbe exact position and 

 character of the absorption-bands ; and before anyone gave evidence 

 on such a subject, that he should make himself practically familiar 

 with the facts I have described. 



In applying this method to the detection of blood-stains on various 

 articles of clothing, it is requisite to ascertain the effect of the different 

 materials, dyes, and mordants. I have for this purpose made a great 

 many experiments, and yet probably much remains to be learned. As 

 a general rule, the dyes exercise little or no prejudicial influence ; but 

 when alum has been used as a mordant, it produces an effect which 

 might easily mislead. Hitherto I have not been able to overcome the 

 effect of the tannin in different sorts of leather. They precipitate the 

 colouring-matter of the blood, and no spectra can be obtained. If, 

 indeed, the blood can be scraped off from the surface without any 

 considerable amount of leather, it can be detected as usual ; but, if it 

 had been sponged or washed, probably no trace whatever could be 

 detected. The same applies in the case of those woods, which, like 

 oak, contain tannic acid. Vegetable soil, when its amount is large in 

 proportion to that of the blood, also precipitates the colouring-matter 

 in an insoluble form ; but, where the relative amount is small, as on 

 dirty clothes, it does not sensibly interfere with the test, though it 

 tends to impair its delicacy. 



When scarlet or brown cruorine is acted on by a solution of alum, 

 it is converted into hsematin. If it has been used as a mordant, 

 sometimes a portion remains in such a condition as to produce this 

 change in blood-stains ; and, moreover, the alumina may combine with 

 the colouring-matter of the blood, and make it insoluble. In such 

 cases it is impossible to obtain the spectrum of either scarlet or brown 

 cruorine ; but if a piece of cloth, mordanted in this manner, marked 

 with blood, be digested in a few drops of water in a watch-glass, and 

 repeatedly squeezed with forceps, a turbid solution is obtained, 

 which, treated with citric acid, filtered, rendered alkaline with 

 ammonia, and deoxidized with protosulphate of iron, shows quite well 

 the spectrum of deoxidized haematin No. 8. 



In a few cases the colouring-matter of the dye is slightly dissolved 

 by water. I have especially noticed this in the case of black printed 

 calico ; and if much of such material, containing very little blood, be 

 digested in water, the dye may cut off the characteristic part of the 

 spectrum. Care should therefore be taken not to use any of the 

 material not well marked with blood, and to have the solution so 

 dilute that the dye may not cut off the green rays. 



Almost everyone must have noticed the fact that, when a blood- 

 stain on white linen or calico has been exposed to pure air for only a 

 short time, it can be almost entirely dissolved out by water ; but, if 

 dried and exposed for some time, the imperfectly soluble brown cru- 

 orine cannot be entirely removed, and leaves a brown stain, which, 

 though readily seen on white material, might be scarcely visible on 

 coloured or black. However, on digesting the suspected portion in a 



