284 



eventually reach a strength where the hypotonic, 

 i.e., haemolysing effect begins to appear. The 

 strength of salt solution just above this where 

 no change occurs is the isotonic point for the 

 particular blood in question. This point then 

 gives us information as to the resistance to a 

 haemolytic action of the corpuscles. The blood 

 in various diseases is found to vary in regard to 

 the strength of salt required to prevent haemo- 

 lysis. So that if a normal blood is unchanged by 

 a o"5 per cent, salt solution, whereas an abnormal 

 requires o-6 per cent, to protect it, the latter 

 blood is described as having a less resistance than 

 the former, but it has a higher isotonic point. 



The determination of the isotonic point then 

 gives us a more definite notion of the state of the 

 blood in disease than does a mere determination 

 of the haemoglobin. The isotonic point of human 

 blood is about 0-41 per cent, salt solution. 



To Determine the Isotonic Point 



1. Measure out one c.c. of each salt solution 

 of descending strengths, 0-43 per cent., 0-41 per 

 cent., 0-39 per cent., etc., into four small test tubes 

 and one c.c. of water into a fifth tube. 



2. Add to each the amount of blood con- 

 tained in two divisions of the stem of a Thoma- 

 Zeiss pipette (the whole stem contains ten divi- 

 sions). 



3. Allow to stand for some time. Some of 

 the solutions will have haemoglobin in solution. 



4. The amount of haemoglobin in each tube 

 can be estimated by adding the amount of a normal 

 blood in two divisions to one c.c. of water. Call 



