172 THE SECRETIONS: 



a short time after the addition of nitric acid, without being 

 first concentrated by evaporation. Indeed Lehmann observed 

 that his morning urine, after living exclusively for five days on 

 animal food, contained so much urea, as to stiffen immediately 

 upon the addition of nitric acid. This might arise either from 

 an absolute increase of urea, or from a relative increase, cor- 

 responding with an augmentation of the solid constituents 

 generally. 



An entire absence of urea has been observed in cases of 

 diabetes insipidus, in which the urine is distinguished by an 

 extreme deficiency of solid constituents : such statements should, 

 however, be received with caution. Willis' instances such 

 cases ; he is, however, inclined to believe that it is always present 

 in very small quantity. In order, therefore, to offer a decided 

 opinion regarding either the absolute pathological increase or 

 decrease of urea, it is requisite to estimate its weight, and the 

 ratio of its weight to that of the solid constituents generally. 

 The method of determining the urea is described at page 136. 

 If the quantity of urea is so small as to render the crystallization 

 of its nitrate imperceptible to the naked eye, the microscope 

 must be used in the manner described when treating of the 

 Blood, in Vol. I, page 182. 



4. Increase, decrease, or absence of uric acid and the urates. 



The variation in the quantity of uric acid in diseases has 

 long been known, but it has not yet been determined with 

 certainty whether this is in all cases an absolute, or whether in 

 some cases it is merely a relative increase dependent upon the 

 increased amount of solid constituents generally. The point 

 must be determined by the quantitative analysis of uric acid, 

 and its ratio to the solid constituents generally. 



a. Increase of uric acid. Urine, containing an excessive quan- 

 tity of uric acid, exhibits in most cases a very high colour, and 

 has an acid reaction. Its specific gravity is frequently lower 

 than would have been supposed from the intense colour. 



1 Urinary Diseases and their Treatment. By Robert Willis, M.D. p. 12. 



