In cases where the amount of fluid obtainable for examination is very small, recourse must be had to the micro-spectroscope, using a Sorby‘s cell to hold the fluid, and substituting for the eye-piece of the microscope a specially constructed spectroscope arranged so as to throw the spectrum of a known solution of blood-colouring matter alongside of that yielded by the solution under examination. Artificial light should be used, and the D line located by placing in the flame a platinum wire carrying a salt of sodium. Biological Tests for Blood The results of the experimental investigations of Friedenthal,[3] Deutsch,[4] Uhlenhuth,[5] Wasserman and Schutze,[6] Nuttall,[7] Tarchetti,[8] Grünbaum,[9] Metchnikoff,[10] and M‘Weeney[11] into “blood relationships” have led to the suggestion of a new method--the “biological test,” by which different kinds of mammalian blood may be distinguished one from the other. Their experiments show in a general way that the “serum from an animal which has been injected intraperitoneally with any given organic fluid will, if mixed in small quantity with a dilute solution of the fluid used for the injection, produce a more or less definite precipitate.” If human defibrinated blood be injected into the peritoneal cavity of a rabbit, the serum obtained from the rabbit‘s blood, when mixed with a clear solution of the blood of man or ape, will produce a precipitate, and agglutinate the red blood corpuscles if present: the same reaction will not follow if such serum be added to a solution of the blood of any other animal. [3] Friedenthal, _Archiv für Anatomie und Physiologie_, 1900, p. 494. [4] Deutsch, _Bulletin Medical_, Sept. 8, 1900; and _Centralblatt für Bakteriologie_, Band xxix. p. 661. [5] Uhlenhuth, _Deutsch Medicinische Wochenschrift_, 1901, Nos. 6, 17, 45. [6] Wasserman and Schutze, _Berlin Klin. Wochenschr._, Feb. 21, 1901. [7] Nuttall, _Jour. Hygiene_, vol. i., 1901, p. 367; _B. M. J._, vol. i., 1901, p. 669. [8] Tarchetti, _Gazz. degli Osped._, May 19, 1901; _B. M. J._, vol. i., 1901. [9] Grünbaum, _Lancet_, Jan. 18, 1902. [10] Metchnikoff, _Centralblatt für Bakteriologie_, April 18, 1901. [11] M‘Weeney, _Jour. State Med._, vol. ix. No. 7, p. 378. _Lancet_, June 18, 1910. The introduction of substances of albuminous or proteid nature into the body of an animal, and which can be taken up by its cells as a food, produces in the body of the animal a series of substances called antibodies, of which one is designated _precipitin_ from its power of producing a precipitate with the substance introduced. The organic substance introduced capable of producing antibodies is called the _antigen_. The precipitin is formed and present in the blood serum of the inoculated animal. Metallic poisons, alkaloids and carbohydrates, do not produce antibodies; some vegetable proteids do. The serum containing the precipitin is called the _antiserum_.