216. It is estimated that the fluids circulating in the adult man amount to about fifty pounds; of these it is calculated that twenty-eight consist of red blood. 217. Fluid and homogeneous as the blood appears while flowing in its vessel, when a mass of it is collected and allowed to stand at rest, it soon undergoes a very remarkable change. First, a thin film is formed upon its surface; this is followed by the conversion of the whole mass into a soft jelly: this jelly separates into two portions, a fluid and a solid portion. The solid portion again separates into two parts, into a substance of a yellowish-white colour, occupying the upper surface, and into a red mass always found at the under surface. 218. The process by which the constituents of the blood are thus spontaneously disunited, and afforded in a separate form, is denominated COAGULATION; the fluid portion separated by the process is termed the SERUM; the solid portion the COAGULUM or CLOT; the white substance forming the upper part of the clot, the FIBRIN; and the red mass forming the under part of it, the RED PARTICLES. 219. Probably the process of coagulation commences the moment the blood leaves its living vessel. In three minutes and a half it is visible to the eye; in seven minutes the mass is formed into a jelly; in from ten to twelve minutes the serum separates from the clot; in about twenty the clot is divided into fibrin and red particles, when the coagulation is complete; but occasionally the clot continues to grow firmer and firmer for the space of twenty-four hours. 220. As soon as the coagulation commences, and during all the time the blood preserves its heat, an aqueous vapour arises from it, termed the HALITUS. The halitus consists of water holding in solution a small quantity of animal and saline matter, which communicate to it a fœtid odour of a strong and peculiar nature, manifest on approaching a slaughter-house, and still more manifest in the slaughter-house of human beings, a field of battle. 221. During the process of coagulation, as in every other in which a fluid is converted into a solid, caloric is evolved. 222. During the process of coagulation carbonic acid is also extricated. 223. The process of coagulation affords three distinct substances, the chief constituents of the blood, namely, serum, fibrin, and red particles.