286 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



gramme of nitrogen ; 100 grammes will therefore contain 11-2 grammes of 

 nitrogen. If the strength of the acid is that just recommended, each c.c. 

 corresponds to 0*0028092 of nitrogen. 



SOLUTIONS. DIFFUSION. DIALYSIS. OSMOSIS. 



The investigations of physical chemists during recent years have given us 

 new conceptions of the meaning of the words that stand at the head of this 

 article. I propose to state what these new conceptions are, and briefly to 

 indicate the bearing they have on the elucidation of physiological problems. 



Solutions. Water is the fluid in which soluble materials are usually 

 dissolved, and at ordinary temperatures it is a fluid, the molecules of 

 which are in constant movement ; the hotter the water the more active are 

 the movements of its molecules, until, when at last it is converted into 

 steam, the molecular movements become much more energetic. Perfectly 

 pure water consists of molecules with the formula H 2 0, and these molecules 

 undergo practically no dissociation into their constituent atoms, and it is 

 for this reason that pure water is not a conductor of electricity. 



If a substance like sugar is dissolved in the water, the solution still 

 remains incapable of conducting an electrical current. The sugar molecules 

 in solution are still sugar molecules ; they do not undergo dissociation. 



But if a substance like salt is dissolved in the water, the solution is then 

 capable of conducting electrical currents, and the same is true for most acids, 

 bases, and salts. These substances do undergo dissociation, and the simpler 

 materials into which they are broken up in the water are called ions. Thus 

 if sodium chloride is dissolved in water, a certain number of its molecules 

 become dissociated into sodium ions, which are charged with positive elec- 

 tricity, and chlorine ions, which are charged with negative electricity. 

 Similarly a solution of hydrochloric acid in water contains free hydrogen 

 ions and free chlorine ions. Sulphuric acid is decomposed into hydrogen 

 ions and ions of S0 4 . The term ion is thus not equivalent to atom, for an 

 ion may be a group of atoms, like S0 4 , in the example just given. 



Further, in the case of hydrochloric acid, the negative charge of the 

 chlorine ion is equal to the positive charge of the hydrogen ion ; but in the 

 case of the sulphuric acid, the negative charge of the SO 4 ion is equal to the 

 positive charge of two hydrogen ions. We can thus speak of monovalent, 

 divalent, trivalent, &c. ions. 



Ions charged with positive electricity are called bat-ions because they 

 move towards the kathode or negative pole ; those which are charged with 

 negative electricity are called an-ions because they move towards the anode 

 or positive pole. The following are some examples of each class : 



Kat-ions. Monovalent: H, Na, K, NH 4 , &c. 



Divalent : Ca, Ba, Fe (in ferrous salts), &c. 



Trivalent : Al, Bi, Sb, Fe (in ferric salts), &c. 

 An-ions. Monovalent : Cl, Br, I, OH, NO.,, &c. 



Divalent : S, Se, So 4 , &c. 



Roughly speaking, the greater the dilution the more nearly complete is 

 the dissociation, and in a very dilute solution of such a substance as sodium 



