6. Reduced hæmatin presents a spectrum with a dark band about midway between D and E, and a broad but paler band commences near the E line and extends to the b line ([Fig. 15, 4]).

Fig. 15.—Blood Spectra.

1. Oxyhæmoglobin.3. Methæmoglobin.5. CO hæmoglobin.
2. Reduced hæmoglobin.4. Reduced hæmatin.6. CO hæmoglobin and
reduced hæmoglobin.

In cases of death by asphyxia, in which the hæmoglobin is in combination with CO₂, the blood, if removed and examined immediately after death, gives a spectrum of reduced hæmoglobin, but on exposure to the air it rapidly changes to oxyhæmoglobin. The period after death at which blood is usually submitted for medico-legal examination is sufficiently late to allow of this change, and so prevents the possibility of determining death by asphyxia by spectroscopic examination of the blood. Where death has been caused by the action of carbon monoxide, the blood is of a cherry-red colour, and it will retain this colour unchanged for a long time, in fact for years, due to a stable combination of the CO and hæmoglobin, called carboxyhæmoglobin. Such blood yields a very characteristic spectrum, with two bands similar to those of O₂Hb, but nearer to the violet end ([Fig. 15, 6]). Their position should be assured by accurate measurement and comparison with a spectrum of O₂Hb. The CO hæmoglobin, however, cannot be reduced; on the addition of ammonium sulphide or Stokes‘ fluid the bands remain unaltered.

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.

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.