PHYSICAL METHODS IX BLOOD EXAMINATION. 255 



Molecular Weight Isotonic Value 



NaCl 58.5 0.585 per cent. 



NaBr 103.0 1.02 



Nal 149-9 1-55 



KNO 3 101.2 i.oi 



KBr 119.1 1.17 



KI 166.0 1.64 



These values while isotonic and isosmotic with each other 

 are not, however, quite isosmotic with the blood. A common 

 salt solution having a percentage strength of 0.9 per cent 

 has practically the same freezing point as the blood. Blood 

 corpuscles (human) in such a solution do not swell or shrink, 

 consequently lose no hemoglobin. But if placed in weaker 

 salt solutions water is gradually absorbed to make the outside 

 and inside osmotic pressure the same. After a time, however, 

 with decreasing strength of the salt solution, so much water 

 is absorbed that the limit of strength of the corpuscle sheath 

 is reached and a break follows. The escape of hemoglobin 

 shows this point. AYith salt solutions this break takes place 

 with practically corresponding osmotic pressures, but there are 

 many substances which do not follow the rule at all. This is 

 particularly true of solutions of urea, glycerol, ammonium 

 carbonate, sodium carbonate and ammonium chloride. Even 

 with rather strong solutions of these bodies the corpuscles fail 

 to hold their hemoglobin. A satisfactory explanation of the 

 abnormal behavior of these bodies is not known. Blood so 

 changed is said to be lake-colored. Following the Hamburger 

 designations normal blood was said to be hyperisotonic, since 

 it contains more than enough salts to hold the corpuscle intact. 



HEMATOCRIT METHODS. 



It has been shown above that the red blood corpuscles main- 

 tain their normal volume in liquids which have the same os- 

 motic pressure as the blood. In liquids with a lower pressure 

 they swell, while in solutions possessing a higher osmotic 

 pressure than the blood they contract. The corpuscles are 

 extremely sensitive to such influences, and changes in volume 



