PHYSIOLOGICAL CHEMISTRY 461 



of these results is that the envelope readily allows water to pass through it 

 but not inorganic salts. The water tends to pass into the corpuscle in the 

 first part of the experiment because the osmotic pressure inside the corpuscle 

 is higher than that outside. To equalise this difference of osmotic pressure 

 water passes in but salts cannot pass out because the membrane is impermeable 

 towards them. The shrinkage of the corpuscle in the second part of the experi- 

 ment bears out this explanation. 



EXPERIMENT II. Determine, ichat strength of NaCl solution just prevents 

 haemolysis. Into each of a series of test tubes place 20 c.c. of sodium 

 chloride solutions of gradually increasing concentration, e.g. 0*5 per cent., 

 0-55 per cent, 0'60 per cent., 0'65 per cent., 070 per cent. 1 To each tube 

 add five drops of fresh defibrinated (ox or dog) blood ; mix by inverting the 

 tube and allow to stand for some minutes. It will be noted that the super- 

 natant fluid in the case of the stronger solutions is colourless, but that in the 

 tube with 0'55 per cent. NaCl it is slightly tinted red, indicating that the 

 corpuscular envelope has ruptured and the haemoglobin has escaped. The 

 saline solution which just prevents haemolysis stands somewhere between 0'55 

 and 0'6 per cent, in strength. 



By estimating the osmotic pressure of blood serum and of the above saline 

 solutions, either by means of the depression of freezing point method or by 

 the microscope or haematocrit (see Exp. IV.), it will be found, however, that 

 a 0'55 per cent, saline solution has a much lower osmotic pressure than that 

 of blood serum (which equals a 0'9 per cent. NaCl solution). The results of 

 the above experiment therefore show us that the corpuscular envelope can 

 withstand a certain amount of hypotonicity before it ruptures. 



If the experiment be repeated with other salts than sodium chloride it will 

 be found that the strength of solution which just fails to show haemolysis 

 bears a close relationship to the molecular weight of the salt used, i.e. the 

 corpuscular envelope gives way at corresponding osmotic pressures. There are 

 certain salts, however, for which this is not true; the most important of 

 these are the ammonium salts and organic substances containing an ammonium 

 residue, e.g. urea, others are sodium carbonate, glycerine, etc. 



EXPERIMENT III. Mix 5 drops of ox blood with 20 c.c. of a 0'7 per cent, 

 solution of ammonium chloride. Laking will occur although this strength of 

 solution has a higher osmotic pressure than a 0'55 per cent, solution of sodium 

 chloride. 



We can determine the osmotic pressure of a saline solution indirectly by 

 observing what effect it has on the volume of red blood corpuscles. When 

 no change in volume occurs with a given solution this must be isotonic with 

 the blood serum ; if it causes swelling it is hypotonic, if shrinkage, hypertonic. 

 Besides employing the microscope to detect these changes in volume, we may 

 employ an instrument called a haematocrit. 



EXPERIMENT IV. The haematocrit consists of two capillary glass tubes, 

 graduated in one hundred equal parts ; the upper ends are widened so as to 



1 These solutions of NaCl are best prepared in the following manner : Fill 

 one burette with a 1 per cent. NaCl solution and another with distilled water. 

 Into test tube 1 place 5 c.c. of the NaCl solution and 5 c.c. of water ( = '5 

 per cent.); into test tube 2 place 5'5 c.c. NaCl solution and 4 '5 c.c. water 

 < = 0'55 per cent.), and so on for the other tubes. 



