Blood Tests

As you are aware, in many criminal cases it is frequently a question of the greatest importance to determine whether stains on the clothing of an accused person, or on a knife or other weapon found in his possession, are or are not due to blood; and further, if such stains are shown to be those of blood, whether they are of human origin. This question is also frequently rendered difficult of solution by the stain being of small dimensions, or old, or on foul linen. The tests which up to quite recently have been relied upon for the determination of these questions are of three kinds-microscopical, spectroscopical, and chemical. Let me here remind you of what blood consists: microscopically it is found to be a transparent, colourless, or faintly yellow liquid, the liquor sanguinis, in which are suspended a number of minute solid bodies known as the blood corpuscles, which are either red or white, but principally the former. When drawn from the body, as seen in these two specimens, the one human and the other sheep's blood, a clotting or coagulation takes place whereby it separates into two distinct parts: (I) a clot, consisting of the fibrin hitherto held in solution, which entangles the cells, which have a tendency to gravitate to the