{ 1. Receives extract of known human blood-stain. { 2. “ “ “ ox “ “ { 3. “ “ “ horse “ “ Set A { 4. “ “ “ sheep “ “ { 5. “ “ “ pig “ “ { 6. “ “ “ stain under observation. Set B is now charged as follows: { 1. Receives normal rabbit serum 0.05 c.c. { 2. “ human antiserum 0.05 “ { 3. “ ox “ 0.05 “ Set B { 4. “ horse “ 0.05 “ { 5. “ sheep “ 0.05 “ { 6. “ pig “ 0.05 “ To each tube in Set B is now added about 0.1 c.c. of the extract of the suspected stain. It will thus be seen that in Set A known antigen is added to known precipitin, with the exception of tube 6, which receives suspected antigen, while in Set B the suspected antigen is added to known antisera, with the exception of No. 1, which contains normal rabbit serum. The tubes are observed in an hour and the results noted. Should the stain be human blood, positive reactions will be present in tubes 1 and 6 of stand A, and tube 2 of B. The negative reactions in the other tubes in stand A prove that the antiserum used is specific, and will only react with its own antigen; in stand B that the stain is composed of specific human antigen, reacting only with its own antiserum. Should the stain suspected be other than human, and derived from one of the other animals used for antiserum in stand B, then the positive reaction will occur in the particular tube.[12] The test is now recognised as very sensitive and reliable, and is also applicable to the detection of the flesh or fluids of animals, and has thus been used for the detection of meat stuffs. [12] M‘Weeney, _Lancet_, June 18, 1910. Tarchetti advises the following procedure in the examination of blood-stains: Dissolve the stain in a few drops of a 0.9 per cent. aqueous solution of sodium chloride, filter, and divide the filtrate into two portions; to one (_a_) add 0.5 c.c. of the prepared rabbit serum (the so-called antiserum); to the other (_b_) serum from a rabbit which has not been injected with human blood. Both are to be placed in an incubator at 37° C. for an hour. By this time, if the solution of the stain be of human or anthropoid origin, the contents of the tube (_a_) will have become turbid, the contents of tube (_b_) will remain clear. From a series of experiments with blood-stains of man and other animals on a variety of materials, Tarchetti states that this method is reliable. Prepared rabbit “human antiserum” has been shown to have no such reaction with the blood of the pig, ox, calf, mouse, or rat.