DIFFUSION AND OSMOSIS 49 



tension of a liquid. The diluted solution is gradually increased 

 in strength until, when a drop of blood is added to it, the 

 corpuscles are just precipitated, and no haemoglobin is 

 dissolved. 



The Hamatocrite. — In 1891, Hedin devised an instrument 

 for determining the influence of different solutions on the 

 red blood corpuscles. This instrument, the haematocrite, is 

 a graduated pipette, designed to measure the volume of 

 the globules separated by centrifugation from a given volume 

 of blood under the influence of the liquid whose osmotic 

 pressure is to be measured. The method depends on the 

 principle that solutions isotonic to the blood corpuscles and 

 to the blood serum will not alter the volume of the blood 

 corpuscles, whereas hypertonic solutions decrease that volume. 



Action of Solutions of Different Degrees qf Concentration on 

 Living- Cells. — We have just seen that a living cell, whether 

 vegetable or animal, is not altered in volume when immersed 

 in an isotonic solution that does not act upon it chemically. 

 When immersed in a hypertonic solution, it retracts ; in a 

 slightly hypotonic solution it absorbs water and becomes 

 turgescent, while in a very hypotonic solution it swells up 

 and bursts. In a hypertonic solution the red blood cells retract 

 and fall to the bottom of the glass, the rapidity with which 

 they are deposited depending on the amount of retraction. 

 In a hypotonic solution they swell up and burst, the haemo- 

 globin dissolving in the liquid and colouring it red. This is 

 the phenomenon of haematolysis. According to Hamburger, 

 the serum of blood may be considerably diluted with water 

 before producing haematolysis. Experimenting with the 

 blood of the frog, he found that the globules remained 

 intact in size and shape when irrigated with a salt solution 

 containing "64 per cent, of salt, this solution being isotonic 

 with the frog's blood serum. On the other hand, they did not 

 begin to lose their haemoglobin till the proportion of salt was 

 reduced to below "22 per cent. Thus frog's serum may be 

 diluted with 200 per cent, of water before producing haema- 

 tolysis. In mammals the blood corpuscles remain invariable 

 in a salt solution of about '9 per cent., and begin to lose their 

 4 



