METHODS OF ELECTRICAL STIMULATION 25 



concentration for the corpuscles of frog's blood is 

 0.6 per cent. 



Surface Tension. The osmotic pressure of 

 solutions is very great. In the ordinary reagents 

 of the laboratory it amounts to many atmos- 

 pheres. Obviously, such solutions could not be 

 kept in thin glass beakers or 

 bottles were these enormous 

 pressures not restrained by 

 some opposing force. This op- 

 posing force is surface tension, 

 which acts with a pressure of 

 probably hundreds of atmos- 

 pheres upon the free surface of 

 a liquid, whether that surface 

 be bounded by air or glass. 

 The semi-permeable membrane 

 is filled with liquid, and there- 

 fore does not present a free 

 surface. 



1. A thick wire is bent to enclose a right- 

 angled space and the end prolonged for a handle. 

 (Fig. 1.) A very fine slack-wire divides the space 

 in halves. 



accurate may be shown by comparison with the method of 

 estimating osmotic pressure by the depression in the freezing- 

 point of the solution, which is proportional to the concentration 

 of the solution (p. 439). 



Fig. 4. The tension 

 indicator. 



