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§ 34. Spots, supposed to be blood—whether on linen, walls, or weapons—should, in any important case, be photographed before any chemical or microscopical examination is undertaken. Blood-spots, according to the nature of the material to which they are adherent, have certain naked eye peculiarities—e.g., blood on fabrics, if dry, has at first a clear carmine-red colour, and part of it soaks into the tissue. If, however, the tissue has been worn some time, or was originally soiled, either from perspiration, grease, or filth, the colour may not be obvious or very distinguishable from other stains; nevertheless, the stains always impart a certain stiffness, as from starch, to the tissue. If the blood has fallen on such substances as wood or metal, the spot is black, has a bright glistening surface, and, if observed by a lens, exhibits radiating fissures and a sort of pattern, which, according to some, is peculiar to each species; so that a skilled observer might identify occasionally, from the pattern alone, the animal whence the blood was derived. The blood is dry and brittle, and can often be detached, or a splinter of it, as it were, obtained. The edges of the splinter, if submitted to transmitted light, are observed to be red. Blood upon iron is frequently very intimately adherent; this is specially the case if the stain is upon rusty iron, for hæmatin forms a compound with iron oxide. Blood may also have to be recovered from water in which soiled articles have been washed, or from walls, or from the soil, &c. In such cases the spot is scraped off from walls, plaster, or masonry, with as little of the foreign matters as may be. It is also possible to obtain the colouring-matter of blood from its solution in water, and present it for farther examination in a concentrated form, by the use of certain precipitating agents (see p. 61).

In the following scheme for the examination of blood-stains, it is presumed that only a few spots of blood, or, in any case, a small quantity, is at the analyst’s disposal.

(1) The dried spot is submitted to the action of a cold saturated solution of borax. This medium (recommended by Dragendorff)[48] does certainly dissolve out of linen and cloth blood-colouring matter with great facility. The best way to steep the spots in the solution is to scrape the spot off the fabric, and to digest it in about a cubic centimetre of the borax solution, which must not exceed 40°; the coloured solution may be placed in a little glass cell, with parallel walls, ·5 centimetre broad, and ·1 deep, and submitted to spectroscopic examination, either by the ordinary spectroscope or by the micro-spectroscope; if the latter is used, a very minute quantity can be examined, even a single drop. In order to interpret the results of this examination properly, it will be necessary to be intimately acquainted with the spectroscopic appearances of both ancient and fresh blood.

[48] Untersuchungen von Blutspuren in Maschka’s Handbuch, Bd. i. Halfband 2.

§ 35. Spectroscopic Appearances of Blood.—If defibrinated blood[49] be diluted with water until it contains about ·01 per cent. of oxyhæmoglobin, and be examined by a spectroscope, the layer of liquid being 1 centimetre thick, a single absorption band between the wave lengths 583 and 575 is observed, and, under favourable circumstances, there is also to be seen a very weak band from 550 to 532. With solutions so dilute as this, there is no absorption at either the violet or the red end of the spectrum. A solution containing ·09 per cent. of oxyhæmoglobin shows very little absorption in the red end, but the violet end is dark up to about the wave length 428. Two absorption bands may now be distinctly seen. A solution containing ·37 per cent. of oxyhæmoglobin shows absorption of the red end to about W.L. 720; the violet is entirely, the blue partly, absorbed to about 453. The bands are considerably broader, but the centre of the bands occupies the same relative position. A solution containing as much as ·8 per cent. of oxyhæmoglobin is very dark; the two bands have amalgamated, the red end of the spectrum is absorbed nearly up to Fraunhofer’s line a; the green is just visible between W.L. 498 and 518. Venous blood, or arterial blood, which has been treated with reducing agents, such, for example, as an alkaline sulphide, gives the spectrum of reduced hæmoglobin. If the solution is equivalent to about ·2 per cent., a single broad band, with the edges very little defined, is seen to occupy the space between W.L. 595 and 538, the band being darkest about 550; both ends of the spectrum are more absorbed than by a solution of oxyhæmoglobin of the same strength. In the blood of persons or animals poisoned with hydric sulphide—to the spectrum of reduced hæmoglobin, there is added a weak absorption band in the red, with its centre nearly corresponding with the Fraunhofer line C. Blood which has been exposed to carbon oxide has a distinct spectrum, due, it would seem, to a special combination of this gas with hæmoglobin; in other words, instead of oxygen, the oxygen of oxyhæmoglobin has been displaced by carbon oxide, and crystals of carbon oxide-hæmoglobin, isomorphous with those of oxyhæmoglobin, may be obtained by suitable treatment. The spectrum of carbon oxide-hæmoglobin, however, differs so little from that of normal blood, that it is only comparison with the ordinary spectrum, or careful measurements, which will enable any person, not very familiar with the different spectra of blood, to detect it; with careful and painstaking observation the two spectra are seen to be distinct. The difference between the carbon oxide and the normal spectrum essentially consists in a slight moving of the bands nearer to E. According to the measurements of Gamgee, the band α of CO-hæmoglobin has its centre approximately at W.L. 572, and the band β has for its centre W.L. from 534 to 538, according to concentration. If a small quantity of an ammoniacal solution of ferrous tartrate or citrate be added to blood containing carbon oxide, the bands do not wholly fade, but persist more or less distinctly; whereas, if the same solution is added to bright red normal blood, the two bands vanish instantly and coalesce to form the spectrum of reduced hæmoglobin. When either a solution of hæmoglobin or blood is exposed to the air for some time, it loses its bright red colour, becomes brownish-red, and presents an acid reaction. On examining the spectrum, the two bands have become faint, or quite extinct; but there is a new band, the centre of which (according to Gamgee) occupies W.L. 632, but (according to Preyer) 634. In solutions of a certain strength, four bands may be seen, but in a strong solution only one. This change in the spectrum is due to the passing of the hæmoglobin into methæmoglobin, which may be considered as an intermediate stage of decomposition, prior to the breaking up of the hæmoglobin into hæmatin and proteids.

[49] In this brief notice of the spectroscopic appearances of the blood, the measurements in wave lengths are, for the most part, after Gamgee.—Text-Book of Physiological Chemistry, London, 1880.

A spectrum very similar to that of methæmoglobin is obtained by treating ancient blood-stains with acetic acid—viz., the spectrum of acid hæmatin, but the band is nearer to its centre, according to Gamgee, corresponding to W.L. 640 (according to Preyer, 656·6). The portion of the band is a little different in alkaline solution, the centre being about 592. Hæmatin is one of the bodies into which hæmoglobin splits up by the addition of such agents as strong acetic acid, or by the decomposing influence of exposure; the view most generally accepted being that the colouring-matter of the blood is hæmatin in combination with one or more albuminoid bodies. The hæmatin obtained by treating blood with acetic acid may be dissolved out by ether, and the ethereal solution then exhibits a remarkable distinctive spectrum. Hence, in the spectroscopic examination of blood, or solutions of blood, for medico-legal purposes, if the blood is fresh, the spectrum likely to be seen is either that of oxyhæmoglobin or hæmoglobin; but, if the blood-stain is not recent, then the spectrum of either hæmatin or methæmoglobin.

The colouring-matter of cochineal, to which alum, potassic carbonate, and tartrate have been added, gives a spectrum very similar to that of blood (see “Foods,” p. 82); but this is only the case when the solution is fresh. The colour is at once discharged by chlorine, while the colour of blood, although changed in hue, remains. The colouring-matter of certain red feathers, purpurin-sulphuric acid, and a few other reds, have some similarity to either the hæmatin or the hæmoglobin spectrum, but the bands do not strictly coincide; besides, no one would trust to a single test, and none of the colouring-matters other than blood yield hæmatin.

The blood in CO poisoning has also other characteristics. It is of a peculiar florid vermilion colour, a colour that is very persistent, lasting for days and even weeks.

Normal blood mixed with 30 per cent. potash solution forms greenish streaky clots, while blood charged with CO forms red streaky clots.

Normal blood diluted to 50 times its volume of water, and then treated successively with yellow ammonium sulphide in the proportion of 2 to 25 c.c. of blood, followed by three drops of acetic acid, gives a grey colour, while CO blood remains bright red. CO blood shaken with 4 times its volume of lead acetate remains red, but normal blood becomes brown.[50]

[50] M. Rubner, Arch. Hyg., x. 397.

Solutions of platinum chloride or zinc chloride give a bright red colour with CO blood; normal blood is coloured brown or very dark brown.

Phospho-molybdic acid or 5 per cent. phenol gives a carmine-coloured precipitate with CO blood, but a reddish-brown precipitate with normal blood (sensitive to 16 per cent.).

A mixture of 2 c.c. of dilute acetic acid and 15 c.c. of 20 per cent. potassic ferrocyanide solution added to 10 c.c. of CO blood produces an intense bright red; normal blood becomes dark brown.

Four parts of CO blood, diluted with 4 parts of water and shaken with 3 vols. of 1 per cent. tannin solution, become at first bright red with a bluish tinge, and remain so persistently. Normal blood, on the other hand, also strikes bright red at first, but with a yellowish tinge; at the end of 1 hour it becomes brownish, and finally in 24 hours grey. This is stated to be delicate enough to detect 0·0023 per cent. in air.

If blood be diluted with 40 times its volume of water, and 5 drops of phenylhydrazin solution be added, CO blood strikes rose-red; normal blood grey-violet.[51]

[51] A. Welzel, Centr. med. Wiss., xxvii. 732–734.

Gustave Piotrowski[52] has experimented on the length of time blood retains CO. The blood of dogs poisoned by this agent was kept in flasks, and then the gas pumped out by means of a mercury pump on the following dates:—

[52] Compt. Rend. Soc. de Biol., v. 433.

Date.	Content of gas in CO.
Jan.	12,	1892,	24·7	per cent.
J „	20,	„	23·5	„
J „	28,	„	22·2	„
Feb.	8,	„	20·3	„
F „	16,	„	15·5	„
F „	26,	„	10·2	„
March	3,	„	6·3	„
Ma„	14,	„	4·6	„
Ma„	22,	„	1·2	„

The same dog was buried on the 12th of January, and exhumed on March 28th, and the gas pumped out from some of the blood; this gas gave 11·7 per cent. of CO; hence it is clear that burial preserves CO blood from change to a certain extent.

N. Gréhant[53] treated the poisoned blood of a dog with acetic acid, and found it evolved 14·4 c.c. CO from 100 c.c. of blood.

[53] Compt. Rend., cvi. 289.

Stevenson, in one of the cases detailed at p. 67, found the blood in the right auricle to contain 0·03 per cent. by weight of CO.

(2) Preparation of Hæmatin Crystals—(Teichmann’s crystals).—A portion of the borax solution is diluted with 5 or 6 parts of water, and one or more drops of a 5 or 6 per cent. solution of zinc acetate added, so long as a brownish-coloured precipitate is thrown down. The precipitate is filtered off by means of a miniature filter, and then removed on to a watch-glass. The precipitate may now be dissolved in 1 or 2 c.c. of acetic acid, and examined by the spectroscope it will show the spectrum of hæmatin. A minute crystal of sodic chloride being then added to the acetic acid solution, it is allowed to evaporate to dryness at the ordinary temperature, and crystals of hæmatin hydrochlorate result. There are other methods of obtaining the crystals. When a drop of fresh blood is simply boiled with glacial acetic acid, on evaporation, prismatic crystals are obtained.

Hæmatin is insoluble in water, alcohol, chloroform, and in cold dilute acetic and hydrochloric acids. It may, however, be dissolved in an alcoholic solution of potassic carbonate, in solutions of the caustic alkalies, and in boiling acetic and hydrochloric acids. Hoppe-Seyler ascribes to the crystals the formula C₆₈H₇₀N₈Fe₂O₁₀2HCl. Thudichum considers that the pure crystals contain no chlorine, and are therefore those of hæmatin. It is the resistance of the hæmatin to decomposition and to ordinary solvents that renders it possible to identify a certain stain to be that of blood, after long periods of time. Dr. Tidy seems to have been able to obtain blood reactions from a stain which was supposed to be 100 years old. The crystals are of a dark-red colour, and present themselves in three forms, of which that of the rhombic prism is the most common (see fig.). But crystals like b, having six sides, also occur, and also crystals similar to c.

If the spot under examination has been scraped off an iron implement the hæmatin is not so easily extracted, but Dragendorff states that borax solution at 50° dissolves it, and separates it from the iron. Felletar has also extracted blood in combination with iron rust, by means of warm solution of caustic potash, and, after neutralisation with acetic acid, has precipitated the hæmin by means of tannin, and obtained from the tannin precipitate, by means of acetic acid, Teichmann’s crystals. A little of the rust may also be placed in a test tube, powdered ammonium chloride added, also a little strong ammonia, and after a time filtered; a small quantity of the filtrate is placed on a slide with a crystal of sodium chloride and evaporated at a gentle heat, then glacial acetic acid added and allowed to cool; in this way hæmin crystals have been obtained from a crowbar fifty days after having been blood-stained.[54]

[54] Brit. Med. Journ., Feb. 17, 1894.

(3) Guaiacum Test.—This test depends upon the fact that a solution of hæmoglobin develops a beautiful blue colour, if brought into contact with fresh tincture of guaiacum and peroxide of hydrogen. The simplest way to obtain this reaction is to moisten the suspected stain with distilled water; after allowing sufficient time for the water to dissolve out some of the blood constituents, moisten a bit of filter-paper with the weak solution thus obtained; drop on to the moist space a single drop of tincture of guaiacum which has been prepared by digesting the inner portions of guaiacum resin in alcohol, and which has been already tested on known blood, so as to ascertain that it is really good and efficient for the purpose; and, lastly, a few drops of peroxide of hydrogen. Dragendorff uses his borax solution, and, after a little dilution with water, adds the tincture and then Heunefeld’s turpentine solution, which is composed of equal parts of absolute alcohol, chloroform, and French turpentine, to which one part of acetic acid has been added. The chloroform separates, and, if blood was present, is of a blue colour.

§ 36. To prove by chemical and physical methods that a certain stain is that of blood, is often only one step in the inquiry, the next question being whether the blood is that of man or of animals. The blood-corpuscles of man are larger than those of any domestic animal inhabiting Europe. The diameter of the average red blood-corpuscle is about the ¹⁄₁₂₆ of a millimetre, or 7·9 µ.[55] The corpuscles of man and of mammals, generally speaking, are round, those of birds and reptiles oval, so that there can be no confusion between man and birds, fishes or reptiles; if the corpuscles are circular in shape the blood will be that of a mammal. By careful measurements, Dr. Richardson, of Pennsylvania, affirms that it is quite possible to distinguish human blood from that of all common animals. He maintains, and it is true, that, by using very high magnifying powers and taking much trouble, an expert can satisfactorily identify human blood, if he has some half-dozen drops of blood from different animals—such as the sheep, goat, horse, dog, cat, &c., all fresh at hand for comparison, and if the human blood is normal. However, when we come to the blood of persons suffering from disease, there are changes in the diameter and even the form of the corpuscles which much complicate the matter; while, in blood-stains of any age, the blood-corpuscles, even with the most artfully-contrived solvent, are so distorted in shape that he would be a bold man who should venture on any definite conclusion as to whether the blood was certainly human, more especially if he had to give evidence in a criminal case.

[55] ¹⁄₃₂₀₀ of an inch; the Greek letter µ is the micro-millimetre, or 1000th of a millimetre, ·00003937 inch.

Neumann affirms that the pattern which the fibrin or coagulum of the blood forms is peculiar to each animal, and Dr. Day, of Geelong, has independently confirmed his researches: this very interesting observation perhaps has not received the attention it merits.

When there is sufficient of the blood present to obtain a few milligrms. of ash, there is a means of distinguishing human blood from that of other common mammals, which has been neglected by authorities on the subject, and which may be found of real value. Its principle depends upon the relative amounts of potassium and sodium in the blood of man as compared with that in the blood of domestic animals. In the blood of the cow, sheep, fowl, pig, and horse, the sodium very much exceeds the potassium in the ash; thus the proportion of sodium oxide to that of potassium oxide in the blood of the sheep is as K₂O ·1 : Na₂O ·6; in that of the cow, as 1 : 8; in that of the domestic fowl, as 1 : 16; while the same substances in human blood are sometimes equal, and vary from 1 : 1 to 1 : 4 as extremes, the mean numbers being as 1 : 2·2. The potassium is greater in quantity in the blood-corpuscles than in the blood serum; but, even in blood serum, the same marked differences between the blood of man and that of many animals is apparent. Thus, the proportion of potash to soda being as 1 : 10 in human blood, the proportion in sheep’s blood is 1 to 15·7; in horse’s serum as 1 to 16·4; and in the ox as 1 to 17. Since blood, when burnt, leaves from 6 to 7 per thousand of ash, it follows that a quantitative analysis of the relative amounts of potassium and sodium can only be satisfactorily effected when sufficient of the blood is at the analyst’s disposal to give a weighable quantity of mineral matter. On the other hand, much work requires to be done before this method of determining that the blood is either human, or, at all events, not that of an herbivorous animal, can be relied on. We know but little as to the effect of the ingestion of sodium or potassium salts on either man or animals, and it is possible—nay, probable—that a more or less entire substitution of the one for the other may, on certain diets, take place. Bunge seems in some experiments to have found no sodium in the blood of either the cat or the dog.

The source from which the blood has emanated may, in a few cases, be conjectured from the discovery, by microscopical examination, of hair or of buccal, nasal, or vaginal epithelium, &c., mixed with the blood-stain.