UREA. 



359 



From these proportions, however, most of the constituents 

 are, even in health, liable to variations. Especially the water 

 is so. Its variations in different seasons, and according to the 

 quantity of drink and exercise, have already been mentioned. 

 It is also liable to be influenced by the condition of the ner- 

 vous system, being sometimes greatly increased in hysteria, and 

 some other nervous affections ; and at other times diminished. 

 In some diseases it is enormously increased ; and its increase 

 may be either attended with an augmented quantity of solid 

 matter, as in ordinary diabetes, or may be nearly the sole 

 change, as in the affection termed diabetes iusipidus. In other 

 diseases, e. g., the various forms of albumiuuria, the quantity 

 may be considerably diminished. A febrile condition almost 

 always diminishes the quantity of water ; and a like diminu- 

 tion is caused by any affection which draws off a large quantity 

 of fluid from the body through any other channel than that 

 of the kidneys, e. g., the bowels and the skin. 



Urea. Urea is the principal solid constituent of the urine, 

 forming nearly one-half of the whole quantity of solid matter. 

 It is also the most important ingredient, since it is the chief 

 substance by which the nitrogen of decomposed tissue and 

 superfluous food is excreted 

 from the body. For its re- FlG - 126 



moval, the secretion of urine 

 seems especially provided ; and 

 by its retention in the blood 

 the most pernicious effects are 

 produced. 



Urea, like the other solid 

 constituents of the urine, ex- 

 ists in a state of solution. But 

 it may be procured in the solid 

 state, and then appears in the 

 form of delicate silvery acicu- 

 lar crystals, which under the 

 microscope, appear as four- 

 sided prisms (Fig. 126). It 18 Crystals of urea. 

 obtained in this state by evapo- 

 rating urine carefully to the consistence of honey, acting on 

 the inspissated mass with four parts of alcohol, then evaporat- 

 ing the alcoholic solution, and purifying the residue by repeated 

 solution in water or alcohol, and finally allowing it to crystallize. 

 It readily combines with an acid, like a weak base ; and may 

 thus be conveniently procured in the form of a nitrate, by add- 

 ing about half a drachm of pure nitric acid to double that quan- 

 tity of urine in a watch-glass. The crystals of nitrate of urea are 



