592 LECTURE XXV. 



example. Suppose we have a solution of ^ normal hydrochloric acid, 

 and one of acetic acid which is also $ normal. From the purely chemi- 

 cal standpoint these solutions are of the same strength, because we shall 

 have to use as much alkali to neutralize a liter of the acetic acid as would 

 be necessary for a liter of the hydrochloric acid. On the other hand, 

 from the point of view taken by the physico-chemist, the acidity of the 

 3*2- normal hydrochloric acid is about 40 times as great as that of the 3 ^ 

 normal acetic acid. Thus the hydrochloric acid of the above concentra- 

 tion is about 97 per cent dissociated, while the acetic acid is only disso- 

 ciated to an extent of 2.4 per cent. Now in our ordinary chemical 

 methods we neutralize all of the hydrogen of the acid, because there is 

 always a fraction of the whole molecule that is dissociated, the value of 

 the fraction increasing with the dilution; and as fast as some of the 

 ions are neutralized more of the molecule dissociates, so that eventually 

 not only the hydrogen ions originally present, what we may designate as 

 the active hydrogen ions, but also those which were originally undissoci- 

 ated, the potential hydrogen ions, are neutralized. The physico-chemist 

 in his determination of the acidity takes into consideration only the 

 former kind of hydrogen. From his point of view the urine is usually 

 neutral. There seems to be no definite relations between the acidity 

 determined by the tit ration of urine and the so-called ion-acidity. It 

 is desirable that in all cases both values should be known. 



In general, not much is known concerning the way in which the dif- 

 ferent substances proved to be present in urine are combined there. The 

 analysis of the ash as such teaches us but little. It gives us considerable 

 information in tracing the course of the non-volatile material, but in this 

 case the intestinal elimination must not be disregarded. The fact that 

 the inorganic substances are, at least to some extent, eliminated by the 

 intestines, complicates our understanding of the general metabolism, and 

 especially because of the fact that in every case it is impossible to decide 

 what part of the constituents of the ash of the faeces is to be re- 

 garded as unabsorbed material and what part was eliminated from the 

 intestinal walls after absorption had taken place. The value of mineral 

 substances for the whole organism and their absolute indispensability 

 have been repeatedly mentioned, and we are convinced that exact investi- 

 gations concerning metabolism on as broad a basis as possible, taking 

 into consideration the inorganic material introduced and that eliminated, 

 will give us considerable information as to the nature of cell-metabolism. 



We must also consider a phenomenon which is frequently met with in 

 human urine. Fresh urine is usually clear and shows no sediment. After 

 the urine has stood for some time a sediment often forms, sometimes as 

 a reddish, crystalline powder, sometimes as a reddish-gray precipitate, 

 resembling brick-dust. The latter is called Sedimentum lateritium. It 



