BLOOD 475 



may be estimated by finding the specific gravity of a solution 

 of sodium sulphate or of chloroform and benzene in which a 

 drop of blood remains where it is placed, neither sinking 

 nor floating. The lowering of the freezing-point of blood or 

 A is 0-o6 C. This is equivalent to the osmotic pressure 

 of a solution of about 0-9 per cent, of NaCl and is the same 

 as that of the cells of the tissues. Viscosity. — The viscosity 

 of blood, or the intermolecular and intermolar friction, may 

 be measured by the time taken to pass through a given 

 length of capillary tube compared by the time taken by 

 water. It depends partly on the viscosity of the plasma, 

 which, being of the nature of an emulsoid colloid, manifests 

 viscosity, but chiefly upon the blood cells. Hence when these 

 are diminished in number the viscosity of the blood is 

 decreased. The Taste and Smell are characteristic, and 

 must be experienced. Reaction. — Blood, so far as the 

 balance of H and OH ions is concerned, is slightly alkaline, 

 its hydrogen ion concentration, Cg, being lower than that of 

 pure water (see Appendix ni.). 



The cells of the blood constitute about 33 per cent., one- 

 third of its weight, and the total sohds of the blood are 

 about 20 per cent. 



II. Clotting or Coagulation, 



Blood, when shed, becomes a firm jelly in the course of 

 three or four minutes. The primary object of the process 

 is to seal wounds in the blood-vessels, and so to prevent 

 hcemorrhage. When the blood is collected in a beaker or 

 other dish, the process starts from the sides, and spreads 

 throughout the blood until, when clotting is complete, the 

 dish may be inverted without the blood falling out. In a 

 short time, drops of clear fluid appear upon the surface of 

 the clot, and, in a few hours, these have accumulated and 

 run together, while the clot has contracted and drawn away 

 from the sides of the vessel, until it finally floats in the clear 

 fluid — the Serum. If clotting occurs slowly, e.g. when the 

 shed blood is cooled, the erythrocytes subside, leaving a 

 layer of clear plasma above, which, when coagulation takes 

 place, forms a " bufty coat " in the upper part of the clot. 



