266 THE PROPERTIES OF PROTOPLASM 



as isotonic those solutions which are just insufficiently Hypertonic 

 to cause withdrawal of water from the cell-contents. The effects of 

 Hypotonic solutions or solutions which are more dilute than the cell- 

 content are more readily studied in cells which possess no rigid support- 

 ing framework, such as the exterior cellulose wall of plant-cells. The 

 Red Blood-corpuscles were first employed by Hamburger for this 

 purpose. If these cells are suspended in sufficiently hypotonic solu- 

 tions, the excessive penetration of water into the cells results in their 

 rupture by the internal pressure which results, and hemoglobin is 

 set free, tingeing the supernatant fluid red. The technique, therefore, 

 consists in suspending the corpuscles in solutions of varying concen- 

 tr'ation and allowing them to settle to the bottom of the tube. A 

 solution which is just sufficiently hypotonic to burst some of the 

 corpuscles will be tinged with hemoglobin and the corpuscles are then 

 said to have undergone Hemolysis. 



The degree of hypotonicity required to rupture red blood-corpuscles 

 is apparently the same for a variety of dissolved substances, so that 

 the solutions are found to be isotonic with one another, as the following 

 data show: 



Limiting concentration 



which causes 

 Substance. Molecular weight. hemolysis, per cent. 



NaCl ,58.5 0.585 



CHsCOOK .....;.... 98.1 1.04 



KNO 3 . . . ....... . .101.1 1.00 



NaBr . . . ... . , 102.9 1.02 



Nal 149.9 1.55 



KI . . . . . . . .... . 166.0 1.65 



Solutions which cause hemolysis, although isotonic with one another 

 are, of course, by no means isotonic with the fluid contents of the 

 corpuscles, for the bursting of the cells indicates not a slight but a very 

 considerable excess of pressure within them. Solutions insufficiently 

 hypotonic to cause actual rupture of the cells will nevertheless cause 

 them to swell through absorption of water, while slightly hypertonic 

 solutions will, on the contrary, cause shrinkage of the cells through the 

 withdrawal of water, just as in the plasmolysis of plant-cells. This is 

 the foundation of the Hematocrit method of measuring isotonicity, 

 devised by Hedin and Koeppe. Blood-corpuscles, freed from serum 

 by washing them with isotonic salt solution, are suspended in measured 

 amounts of various solutions to be investigated, and the mixtures are 

 placed in specially constructed centrifuge-tubes of very narrow bore 

 and provided with fine graduations. The tubes are then centrifuged 

 and the heights of the columns of corpuscles compared in the various 

 tubes. If the corpuscles have swollen they will occupy a larger volume 

 in the tube, if they have lost water they will occupy a smaller volume 

 than the corpuscles which are immersed in strictly isotonic salt solu- 

 tions. From the lowering of the freezing-point we know that blood- 

 serum is isotonic with NaCl or f sugar solutions, and it is experi- 

 mentally found that in the majority of instances salt solutions which 



