As already stated, hæmoglobin by exposure to air or by the action of reagents is changed into hæmatin. Hæmatin, being insoluble in water, yields crystals only from its solution in acid. Teichmann in 1853 found by treating dried blood with strong acetic acid in presence of common salt that small crystals entirely different from hæmoglobin were produced. They have been found to be hæmatin hydrochloride, and are commonly called hæmin, and sometimes Teichmann's crystals. (Zeitschrift fur rat. Med., Zurich, vol. iii., p. 375.) Most minute traces of blood will yield these crystals, and all authorities agree that they can be produced from no other substance. They assume the form of slender prisms with irregular rhombic terminations. They are frequently found in stellate groups and in the form of an X. They vary considerably in size, according to the strength of the solution, and are identical in composition and crystalline form in all of the different kinds of blood which have been examined. Hæmin crystals are obtained by heating a drop of fresh or a small particle of dried blood with a trace of salt and glacial acetic acid. The materials are placed in the center of a microscope-slide and heated until by the evaporation of the acid the liquid begins to solidify. In some cases the microscope shows transparent cubical crystals mixed with the crystals of hæmin. These are due to the use of an excess of salt, but the two kinds of crystals are readily distinguished by their difference in form, color, and solubility.

(Halliburton, Text-book of Chemical Physiology, London, 1891.) (Gamgee, Physiological Chemistry.)

The Action of Chemical Agents upon Blood.-Water-Fresh blood mixes freely with cold water, and forms a bright-red solution. The color, as already stated, is due to hæmoglobin. By continued exposure this body becomes less and less soluble, owing to the production of met-hæmoglobin, and finally, when converted into hæmatin, the bloodcoloring matter is quite insoluble in water. The action of water upon blood-stains is therefore variable according to the age of the stain. Comparatively recent stains are soluble, older ones less so, and very old stains may be wholly insoluble. Sometimes the latter may be soaked in water for weeks without the liquid becoming in the least degree colored.

Ammonia and the Alkalies.-Ammonia added to a solution of blood, if in small amount, has no effect upon the color except to make it a little brighter and clearer. If a larger quantity or a very strong solution be employed, the red color becomes brown. Solutions of caustic potassa or soda produce a dirty green color with stains on linen. Old stains upon cloth or portions of dried blood are dissolved by dilute ammonia water on soaking, and especially by the aid of a gentle heat. Solutions of potassa or soda acting upon stains give them a darker color before they are fully dissolved.

Effect of Heat.-A solution of blood is coagulated by boiling. On continuing the heat, a more or less voluminous precipitate, according to the strength of the solution, makes its appearance. The precipitate is reddish brown in color, and consists of albumen rendered insoluble by the heat, combined with the coloring matter of the blood. The precipitate is readily soluble in ammonia water.

Bleaching Agents.-Chlorin water, solution of sulphurous acid, and solution of sodium hypochlorite (the so-called chloride of soda) have but little effect upon the color of blood unless the solutions are concentrated

or heated with the blood. Acids of all kinds coagulate the albumen of blood, which, on precipitation, carries down the coloring matter.

Tincture of Galls.-A tincture of galls or an alcoholic solution of gallic acid produces a reddish precipitate, containing the albumen and coloring matter.

The reactions of ammonia, bleaching agents, and of tincture of galls are peculiar to blood. All other soluble red colors are differently affected. They are changed to a green, blue, purple, and in some cases a crimson color by dilute ammonia. Vegetable colors are readily bleached, and certain compounds of iron which are reddish brown in color are changed by the tincture of galls to a bluish black or a bluish green.

Alcohol, Ether, Chloroform, etc.-Dried blood is insoluble in strong alcohol, ether, petroleum benzin, or chloroform. The coloring matter of dried blood is soluble in nearly all of the acids.

Tincture of Guaiacum.-When a small quantity of a freshly made solution of guaiacum resin in alcohol is added to blood either in a pure state or mixed with a large amount of water, there is a precipitation of the resin which renders the liquid milky white. If now a few drops of hydrogen peroxide be added, there is at once produced a beautiful sapphire-blue coloration of the liquid. Particles of dried blood or stains upon cloth give the same reaction.

The Blood-Corpuscles.-When a very thin film of fresh blood is examined by the microscope under a power of two hundred diameters, it is seen to consist of a clear and nearly colorless fluid, in which are suspended an innumerable number of isolated cells having a reddish-yellow tint, called blood-corpuscles. Seen flatwise they appear as disks, while edgewise they show a depression on each of the sides, which gives them somewhat the appearance of a thin rubber ball compressed in the central portion by pressure between the fingers.

Corpuscles were first observed by Malphigi in 1661 in the blood of the hedgehog, but he considered them merely globules of fat. Leeuwenhoeck in 1673 detected them in human blood. The early observers of these bodies thought them spherical in form, and called them bloodglobules; but Hewson in 1770 showed that they are not spheres, but disks. Young in 1813 inferred that their flat surfaces are depressed in the center, but the fact that they are bi-concave disks was only finally determined by Hodgkin and Lister in 1827. Further study of the blood showed that it contains two other kinds of corpuscles-white corpuscles and the so-called blood-plates of Hayem, discovered in 1878.

The red corpuscles are by far the most numerous; the white ones are found (in normal blood) only in the small proportion of one to about five hundred of the red, and the blood-plates to the extent of about one to twenty of the others. Neither the white corpuscles nor the bloodplates have any importance in the examination of blood-stains, and it is unnecessary to consider them further.

A comparative study of the red corpuscles of the vertebrates shows considerable difference both in size and form of these bodies as they exist in the various classes. With a few unimportant exceptions, the corpuscles of the mammalia are circular, while those of the ovipara are oval. In certain of the ruminantia, including the camei, dromedary, llama, and alpaca, the corpuscles are oval; in the Mexican deer they have

a variety of forms-circular, heart-shaped, crescent, etc., even in a single individual. (Gulliver.) The lamprey and a few fishes of the cyclostoma have circular disks, which, like those of the mammalia, are slightly depressed in the center. In the corpuscles of all of the ovipara there is found an aggregation of granules called a nucleus, which gives them a raised center. The addition of acetic acid to such corpuscles renders the surrounding parts more transparent and makes the nucleus more distinctly visible. In all of the mammalia there is an absence of this nucleus, and the corpuscles have a depressed center. Under certain conditions of light and focus circular corpuscles appear to have a dark center, but this is the effect of refraction due to the lenticular form of the disk. Fig. 13.

shows the appearance of the round and of the nucleated oval corpuscles. The round corpuscles at 1 are represented as being sharply focused, while at 2 they are shown within the focus. Just beyond the focus they have the center dark and the periphery bright. The effect of acetic acid upon the nucleus is seen at the extreme right of the figure.

The size of the corpuscles varies considerably in the different classes, and is variable within smaller limits in animals of the same class, and even in the same animal. The diameters of the corpuscles bear no relation to the size of the animal. They are larger in the mouse than in the lion, while the ox and the horse have corpuscles considerably smaller than those in man. The largest corpuscles are found among the batrachia; the long diameter of the corpuscles of the eel-salamander (amphiuma) is 1-350 of an inch. The smallest corpuscles are found in the musk-deer. In this animal their average diameter is 1-12,000 of an inch. The corpuscles of the reptiles are the largest; next in size are those of fishes and birds. Mammalian corpuscles are the smallest of all. Among the latter class a few genera have corpuscles somewhat larger than man, but in the majority they are smaller. Gulliver made careful measurements of the diameters of the corpuscles in nearly six hundred genera. His tables may be found in the Proceedings of the Zoological Society of London (1875, p. 474). All of Gulliver's measurements, with some additions, are given in Milne-Edwards' Treatise on Comparative Physiology. (Lecons sur Physiologie, etc., Paris, 1857, vol. i., pp. 83-90.)

The number of red corpuscles found in a single drop of blood is very large. Five million of these bodies, according to Vierordt and other authorities, is the estimate of the number contained in a cubic millimeter (one twenty-fifth of an inch)-a volume not larger than the head of a small pin.

The structure of the red corpuscles is generally believed to consist of a network of albuminous fibers attached, according to some observers, to the outer hardened layer of the protoplasm of the corpuscle, and according to others to a cell wall or investing membrane. The cell thus formed holds a fluid which contains the blood-coloring matter, or hæmoglobin. In consequence of this structure the corpuscles exhibit the phenomena of endosmose and exosmose. If placed in water—a liquid of less specific gravity than that contained in the corpuscles-the water penetrates into their interior, increases their volume, and thereby causes them to become spherical. This change is necessarily accompanied by a reduction in diameter amounting to about one third. A human blood-disk having a diameter of 1-3300 of an inch if placed in water becomes a sphere of about 1-4500. The oval corpuscles of the ovipara act in the same manner, and, except for the presence of the nucleus, appear like those of mammalian blood. On the other hand, if corpuscles are immersed in a liquid of high specific gravity, as a solution of sugar or of sodium sulphate, the fluid in the interior passes into the outer liquid, and the sides are brought closely together and the corpuscle becomes shriveled and the edges crenated. Weak solutions of the alkalies and dilute acids dissolve the substance of the corpuscle and speedily destroy it. Concentrated solutions of the alkalies (thirty-three percent.) act much more slowly, and form a very good medium for separating the corpuscles from a mass of dried blood under microscopic examination. A solution of common salt, glycerin, or other substance not capable of acting upon albumen, if of specific gravity substantially the same as that of the serum of the blood, may be mixed with the blood without producing any distortion or dissolving effect upon the corpuscles.

THE PRACTICAL TESTING OF SUSPECTED STAINS.

Preliminary Examination.-Before proceeding to the actual tests, every article of clothing, instrument, or other object upon which stains are suspected should undergo a minute inspection, and memoranda of the details put in writing, as follows:

(1) Date and time of day when the specimens were received.

(2) From whom the specimens were received, together with all particulars as to condition of the packages, seals (if used), place where the specimens were received, etc.

(3) A detailed list indicating every article submitted for examination. (4) The number, size, shape, and exact position of the suspected stains.

(5) If the stains are upon clothing or other fabric, note the side on which they occur. Examine particularly all the pockets, linings, buttons, and seams. These portions often contain minute clots which escape

casual observation.

(6) If there appear to be drops, spatters, or smudges upon any of the objects, note the direction of the spatters and the appearance of the drops. (See remarks below on direction of spatters, etc.)

Observe also the following precautions:

(1) When not in use, keep the specimens under lock and key.

(2) As the examination proceeds, each spot tested should be marked.