The blood of the bullock, sheep, and pig is very difficult to crystallise. By the method adopted by Gamgee of adding to defibrinated blood one-sixteenth its volume of ether, shaking until the mixture becomes transparent, and allowing to stand in an ordinary temperature for 48 hours, crystals may be obtained from the blood of the following animals: 1. Horse. 2. Bullock. 3. Sheep. 4. Pig. 5. Dog. 6. Cat. 7. Rabbit. 8. Squirrel. 9. Guinea-pig. 10. Rat. 11. Mouse. 12. Chicken. 13. Pigeon. Crystals from human blood are not easily obtainable by this process, but when they are, they always give the spectrum of reduced hæmoglobin, whereas those from the animals mentioned above give the spectrum of oxyhæmoglobin. Chemical Examination Having obtained a coloured solution from a supposed blood-stain, if sufficient in quantity, apply the following chemical tests to separate portions: 1. Add a few drops of a weak solution of ammonia in distilled water. The colour may remain unchanged, or, at the most, a _slight_ heightening may take place, if it be due to blood. If the solution of ammonia be too strong, a brown colour may be produced if blood be present. 2. Heat to boiling, when the following changes take place if blood be present: (_a_) The colour may disappear. (_b_) Coagulation follows. (_c_) A precipitate falls, dirty grey or brown in colour, depending upon the amount of colouring matter present. On adding caustic potash to the precipitate it will dissolve, and the solution formed will appear greenish by transmitted and red by reflected light. This phenomenon is called the _dichroism_ of blood. Authorities differ in opinion as to whether the colour is green by transmitted and red by reflected light, or _vice versa_. “As a matter of fact, the phenomenon is chameleon-like as regards colour, so that both sets of observers may be considered right or wrong” (Glaister). [Illustration: Fig. 10.--Photo-micrograph of red blood corpuscles from domestic fowl, × 250. (R. J. M. Buchanan.)] [Illustration: Fig. 11.--Photo-micrograph of blood corpuscles of fish, × 250. (R. J. M. Buchanan.)] [Illustration: Fig. 12.--Photo-micrograph of blood corpuscles from a dried stain of the blood of a codfish, × 250. (R. J. M. Buchanan.)] 3. Add tincture of guaiacum, freshly prepared: an opaque, cream-coloured precipitate of the guaiac resin will form in the aqueous solution. On the addition of ozonic ether, turpentine, or peroxide of hydrogen, a blue colour will be produced at the junction of the fluids: proportionate to the amount of blood-colouring matter present, the blue colour will vary in intensity. This test, known as Day‘s or Schönbein‘s, is extremely delicate, and _reacts to no coloured substance except blood_.