28 PHYSIOLOGY FOR DENTAL STUDENTS. 



pand in the solvent, that is, to take up more room by attracting 

 the solvent through the membrane. Cell membranes are semi- 

 permeable, but they are too small and delicate for experimental 

 purposes, for which we use one composed of a precipitate of 

 copper ferrocyanide supported in the pores of an unglazed clay 

 vessel. If a solution of -a crystalloid say, cane sugar be placed 

 in such a vessel and this then submerged in water, it will be found 

 that the cane sugar solution quickly increases in volume, or, if 

 this be prevented by closing up the vessel and connecting a pres- 

 sure gauge with it, a remarkably high pressure will become devel- 

 oped. This is called osmotic pressure, and it is a measure of the 

 tendency of dissolved crystalloids to expand in the solvent. It 

 has been found that the laws which govern osmotic pressure are 

 identical with those governing the behavior of gases. Therefore, 

 the osmotic pressure would be expected to be proportional to the 

 number of molecules of dissolved crystalloid and such is the case 

 for the sugars, but it is not so for the saline crystalloids, such as 

 the alkaline chlorides, nitrates, etc. These cause a greater 

 osmotic pressure than we should expect from their molecular 

 weights. Why is this ? The answer to the question is revealed by 

 observing the behavior of the two classes of crystalloids towards 

 the electric current. Solutions of sugars or urea do not conduct 

 the current any better than water, whereas solutions of saline 

 crystalloids conduct very readily. The former are therefore 

 called non-electrolytes and the latter electrolytes. The reason for 

 these differences has been found to be that molecules of electro- 

 lytes when they are dissolved break into parts called "ions," 

 and each ion carries a charge of electricity of a certain sign, i. e., 

 positive or negative. Whenever an electric current is passed 

 through the solution, the ions, hitherto distributed throughout 

 the solution in pairs carrying charges of opposite signs, now line 

 themselves up so that the ions with one kind of electrical charge 

 form a chain across the solution along which that kind of elec- 

 tricity readily passes, and in so doing carries the ions with it. 

 This splitting of electrolytes into ions is called dissociation or 

 wnization. The ions which carry a charge of positive electricity 

 and which therefore travel towards the kathode or negative pole 



