200 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



^ndicafor of the end of the reaction. (This gives a pink colour with acid,. 



yellow with alkaU.) 



Exmnple. — Suppose 0*15 gramme of a nitrogenous substance is taken,. 



treated with acid, neutralised, and the ammonia distiUed over and received by 



100 c.c. of a decinormal solution of hydrochloric acid (=10 c.c. normal acid). 



The distillate is then titrated with decinormal soda, and it is found that the 



neutral point is reached when 60 c.c. of the decinormal soda have been added. 



The other 40 c.c. must therefore have been neutralised by the ammonia 



derived from the nitrogenous substance under investigation. This 40 c.c. of 



decinormal acid = 4 c.c. of normal acid = 4 c.c. of normal ammonia = 4 



X 0*017 = 0'068 gramme of ammonia ; 0*15 gramme of the substance 



therefore yields 0'068 gramme of ammonia, and this amount contains 0*056 



gramme of nitrogen : 100 grammes of the substance will therefore contain 



100 X 0*056 



q:t~R = 37*3 grammes of nitrogen. 



Fig. 82 represents the apparatus as modified by Willfarth. In this, 

 oxidation is assisted by adding a small quantity of metallic mercury (about 

 ■^ c.c). To avoid bumping during distillation talc is added instead of zinc. 

 12 c.c. of strong potassium sulphate solution are also added. Decinormal 

 sulphuric acid is used as the standard acid, and this is contained in the bulbs 

 shown in the figure. 



When this method is used for determining the total nitrogen in urine, 5 c.c. 

 of urine and 20 c.c. of the mixed acids are boiled for half an hour in a flask 

 of 300 c.c. capacity. After cooling, this is distilled with soda, and the process 

 completed as already described. 



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 our new ideas have^been principally worked out in connection 

 with this fluid. Water at ordinary temperatures is a fluid the molecules of 

 which are in constant movement ; the hotter the water the more active ai*e 

 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.iO, 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 



