tive of uric acid. The contents of the capsule are next in-` cinerated, the residue treated with a few drops of water, and the liquid divided into two portions. One is slightly acidified with acetic acid and tested with a drop of oxalate of ammonium, when the production of a white turbidity will indicate the presence of calcium. The other is acidified with hydrochloric acid and evaporated cautiously to dryness, when the production of microscopic cubical crystals will show the presence of sodium.

7. Acid solution. By means of dilute ammonia, the solution B is made as nearly neutral as it can be, without having its transparency affected. Acetate of ammonia is then added, the production by which of a white precipitate will indicate the presence of oxalate of calcium, or cystine. The latter body rarely occurs in mixed calculi, and could be readily separated from the oxalate by treatment with ammonia: on evaporating the ammoniacal solution, it would be deposited in the form of hexagonal tablets. To the clear liquid, if no precipitate has formed, or otherwise to the filtrate therefrom, oxalate of ammonium is added in excess, when the deposition of a white precipitate will indicate the presence of calcium, which did not previously exist in the state of an oxalate. Filtration is next performed, if necessary, and an excess of ammonia added to the clear liquid, when the production of a white crystalline precipitate, after stirring for a little while, will prove the presence of phosphoric acid and of magnesium. Should there be no obvious precipitate, sulphate of magnesium is to be added, when the presence of phosphoric acid will be indicated by the formation, after brisk stirring, of a white crystalline precipitate.

d. Insoluble residue. The residue C is to be treated with concentrated nitric acid, and the whole evaporated to dryness, whereby a pink mass will be left, which held over the vapour of ammonia will become crimson, and if subsequently moistened with potash will become purple, reactions characterising uric acid.

§ VII.-BLOOD.

(173.) COAGULATION.

a. Blood as existing in the vessels is seen to consist of red corpuscles floating in a clear liquid, termed the liquor sanguinis. When removed from the living vessels it speedily separates into two portions, a clear yellow liquid, the serum, and a solid red mass, the clot. The liquor sanguinis consists of fibrin and serum; the clot, of fibrin and corpuscles, as seen in the following diagram:

Thus the chemical investigation of the blood naturally divides itself into separate examinations of the clot and serum.

6. The coagulation of blood is due to the solidification of fibrin, which entangles in its meshes the corpuscles and a considerable portion of the serum, so as to form a firm jelly-like mass. While the blood is circulating through the vessels of living animals, the fibrin exists in a state of perfect solution. The circumstances which determine this state of solution are not well understood; but intimate contact with the living tissues appears to be one very important condition. Out of the body the fibrin speedily solidifies, the coagulation, which is accompanied by a slight evolution of ammonia, being generally complete in about ten minutes' time. Variations of temperature, movement, and exposure to air, modify but never prevent the coagulation. Where the fibrin exists in large quantity, the coagulation takes place more slowly, but the coagulum is firmer and more compact. When blood is removed from persons suffering from an inflammatory condition of system, or when it contains an excess of fibrin, or a deficiency of corpuscles, or when it is collected in a deep narrow vessel, or when its coagulation is retarded by any means, the corpuscles sinking before the coagulation is complete,

exist principally in the lower portion of the clot, while the upper layer consists of nearly colourless fibrin. This colourless layer is termed the buffy coat; it is extremely tenacious, and frequently by its slow contraction draws up the edges of the clot, so as to form a cup-like depression.

(174.) FIBRIN.

a. From liquid blood. Fibrin is most easily procured from this source. The blood, before it has had time to coagulate, is rapidly whipped with a few twigs of wood, or well shaken in a bottle with two or three irregular pieces of lead. In this way the fibrin separates more or less completely from the corpuscles, and adheres to the twigs or pieces of lead in the form of loose fibrous masses. These are to be well washed with water, and also with ether, when it is desired to remove the adherent fat.

6. From the clot. The preparation of fibrin from coagulated blood is rather more tedious. The clot should be placed upon a cloth, thoroughly broken up by the hand, and washed under a stream of water; when, by alternate washing and kneading, the serum and colouring matter of the clot will pass through the cloth, and a residue of tolerably white fibrin be left thereon.

y. Properties of fibrin. Fibrin possesses all the chemical properties of coagulated albumen (vide par. 176 y). When examined microscopically it is seen to differ from coagulated albumen in manifesting an organised structure, though of the lowest type, viz., the simply fibrous. This fibrillated arrangement is best seen in the buffy layer of inflammatory blood. When moist fibrin, especially that obtained from the clot, is covered with water rendered faintly alkaline by soda, and left at rest for some days in a tolerably warm.situation, the greater part of it dissolves, and albumen may be detected in the filtered liquid by the action of heat and nitric acid. Fibrin constitutes about O'25 per cent. of normal blood.

(175.) CORPUSCLES.

Fig. 68.

a. Their appearance. When a drop of uncoagulated blood, or a drop of the deep red-coloured serum squeezed out of the clot, is examined under a good quarter-inch object glass with a high eye-piece, the field of the microscope is seen covered with minute coloured cells, of uniform size, circular outline, and nongranular structure, as shown in fig. 68. According to the focussing the edges will appear dark and the centre transparent, or vice versa. Some of the globules may be seen lying upon their edges, some of them adhering to one another by their flat surfaces, forming rouleaus. In the case of the previously uncoagulated blood, a delicate net-work of

fibrin will speedily appear. Blood corpuscles appear to consist of a transparent membrane containing a red-coloured fluid. The phenomena of osmose may be readily seen under the microscope: thus if a concentrated solution of sulphate of sodium be added, the corpuscles become distorted, their edges uneven, and their dark centres more prominent; if, however, water be added, the corpuscles swell up, their dark centres and defined margins gradually disappear, and finally the cells burst with discharge of their contents.

In addition to the above-described red corpuscles, there may generally be seen a few of the colourless or lymph corpuscles. In healthy blood, these exist in a variable but very small proportion compared with the red, than which they are rather larger in size, and less uniform in outline. Moreover, they manifest a faintly granular structure.

B. Their separation. If blood as it is flowing be received into a saturated solution of sulphate of sodium, all

coagulation will be prevented, and by repose the corpuscles will form a bright scarlet layer at the bottom of the vessel. The supernatant fluid may be poured off, and the sediment collected in a filter, and washed with a solution of sulphate of sodium. Or the red liquor, from which the fibrin has been removed by agitation, may be allowed to subside; or the clot may be broken up, well shaken with the serum, and the red fluid so formed be allowed to subside. From either of these fluids a gradual but incomplete deposition of the corpuscles will take place. The supernatant serum may then be poured away, and replaced by a solution of sulphate of sodium, when the corpuscles will behave as in the first instance, and may be collected upon a filter and washed with sulphate of sodium as before.

y. Red colouring matter. Hæmatosine, or the red colouring matter of blood, is remarkable for the amount of iron which it contains. The ash of blood corpuscles yields fully 30 per cent. of peroxide of iron. The presence of iron in any of the other tissues or fluids, with the exception of the chyle, appears to be due to an admixture of blood. It is possible indeed to obtain a modified hæmatosine free from iron; but no inference can be drawn from the experiment. The chemical reactions of the colouring matter may be recognised by throwing the corpuscles, well washed with sulphate of sodium and drained, into a considerable excess of cold water, when the cell walls burst by endosmosis, and the coloured contents of the cells dissolve in the water, forming a deep-red solution, which by filtration may be made perfectly bright. This red colouring matter is unaffected by ammonia, and is entirely destroyed by ebullition, with the formation of a dirty-coloured coagulum, which dissolves in caustic potash with an indistinct greenish colour.

d. Hæmatocrystallin, or blood crystals. The formation of these crystals was first discovered by Dr. Otto Funke, of Leipsic. It appears that by the bursting of the red corpuscles as above described an aqueous solution of their contents is obtained, which by very slow evaporation yields crystals