28 A TEXTBOOK OF PHYSIOLOGY 



case of many aquatic vertebrates this is not the case, the blood 

 under ordinary conditions has a different osmotic pressure to the 

 medium in which the animal lives, and" is but slightly altered by 

 variations in the medium. This is true, for instance, of the tcleostean 

 fishes ; the blood of the elasmobranchs, on the other hand, varies in 

 osmotic pressure with that of the surrounding sea-water. 



It was stated above that the lowering of the osmotic pressure of 

 a solution is proportional to the concentration of the dissolved sub- 

 stance. Although this is true, it is found that there are very many 

 substances, such as sodium chloride, for example, which yield, by the 

 method of the lowering of the F.P., a molecular weight quite incon- 

 sistent with the formulae accepted for them. The osmotic activity 

 of these bodies points to an abnormally large number of dissolved 

 units in their solutions. This is explained bj- the view that acids, 

 bases, and salts, in aqueous solution become dissociated to a greater 

 or less extent into positively and negatively charged particles or ions. 

 These ions increase the number of units present in the solution, and 

 endow it with an enhanced osmotic activity. Sodium chloride, for 

 example, when dissolved in water splits to a large extent into positively 



+ 



charged sodium ions, Na, and into negatively charged chlorine ions, 



+ + 



Cl. Hydrochloric acid splits into H and Cl, caustic potash into K 



H- 



and OH, potassium nitrate into K and N0 3 . One molecule, it will 

 be seen, produces but two ions. The " ionic " hypothesis furnishes 

 an adequate explanation of the abnormal osmotic influence exerted 

 by such bodies ,'n aqueous solution. It also explains intel- 

 ligibly the behaviour of various solutions to the passage of an 

 electric current. It is known that the solutions of the bodies which 

 give an abnormal effect in lowering the F.P. of water ako conduct 

 an electric current: they are electrolytes. When two electrodes, one 

 charged positively and the other negatively, are placed in such solu- 

 tions, according to this hypothesis an attractive force is exerted 

 upon the ions of opposite signs. Thus, the positively charged ions 

 move towards the negative electrode, and the negatively charged to 

 the positive electrode ; the undissociated neutral molecules, remaining 

 unaffected and exhibiting no tendency to move in either direction, 

 play no part in the transport of electricity through the solution. 

 The efficiency, thej'efore, of a given quantity of a salt to conduct an 

 electric current depends upon the extent of dissociation of that salt. 

 It is found by experiment that the amount of dissociation, and there- 

 fore the conductivity, increases as the solutions of the salts become 

 less concentrated. 



In the body fluids there are some substr.nccr; in solution w r hich are 

 electrolytes, and will therefore conduct electricity; others which are 

 non-electrolytes, and will not conduct electricity. The fluids conduct 

 according to the amount of the electrolytes present. Thus, blood- 

 serum has a conductivity of about the same as that of a 0-8 per 

 cent, sodium chloride solution. When the non-conducting corpuscles 



