414 



EXCRETION. 



The formula for urea, showing the presence of a large proportion of nitrogen, would 

 lead us to suppose that this substance is one of the products of the waste of the nitrogen- 

 ized principles of the body. It is found, under normal conditions, in the urine, the lymph 

 and chyle, the blood, the sweat, and the vitreous humor. Its presence has lately been de- 

 monstrated, also, in the substance of the healthy liver in both carnivorous and herbivorous 

 animals ; and it has farther been shown by Zalesky that it exists in minute quantity in 

 the muscular juice. Under pathological conditions, as has been already intimated, urea 

 finds its way into various other fluids, such as the secretion from the stomach, the serous 

 fluids, etc. 



In connection with the chemical properties of urea, it is interesting to note that it i& 

 one of the few organic proximate principles that can be produced synthetically in the 

 laboratory of the chemist. As early as 1828, Wohler obtained urea by adding sulphate 

 of ammonia to a solution of cyanate of potassa. The products of this combination are 

 sulphate of potassa, with cyanic acid and ammonia in a form to constitute urea. The 

 cyanate of ammonia is isomeric with urea, and the change is effected by a simple re- 

 arrangement of its elements. It has long been known that urea, in contact with certain 

 animal substances, is readily convertible into carbonate of ammonia. This transformation 

 is theoretically accomplished by adding to urea four atoms of water. It has recently been 

 stated by Kolbe, that carbonate of ammonia, when heated in sealed tubes to the tem- 

 perature at which urea commences to decompose, is converted into urea. The decom- 

 position of urea resulting in the carbonate of ammonia may be easily effected by various 

 chemical means. As this occurs in the spontaneous decomposition of urea in the urine 

 and elsewhere, it has been supposed that the symptoms of blood-poisoning following re- 

 tention of the urinary constituents, in cases of disease of the kidneys, are due to the 

 decomposition of the urea into carbonate of ammonia, and not to the presence of the urea 

 itself in the blood. Many interesting experiments and observations have been made upon 

 this subject, but it is now pretty generally admitted that the weight of evidence is against 

 the carbonate-of-ammonia theory of uraemia. 



Except as regards the probable changes that take place in the process of transforma- 

 tion of certain constituents of the tissues into urea, the chemical history of this substance 

 does not present much physiological interest. Urea may be readily extracted from the 

 urine, by processes fully described in all the modern works upon physiological chemistry ; 

 and its proportion may now be easily estimated by the new methods of volumetric anal- 



ysis. It is not so easy, however, to separate it 

 from the blood or the substance of any of the 

 tissues, on account of the difficulty in getting 

 rid of the other organic matters and the great 

 facility with which it undergoes decomposi- 

 tion. 



When perfectly pure, urea crystallizes in 

 the form of long, four - sided, colorless, and 

 transparent prisms, which are without odor, 

 neutral, and in taste resemble saltpetre. These 

 crystals are very soluble in water and in alco- 

 hol, but they are entirely insoluble in ether. 

 In its behavior to reagents, urea acts as a base, 

 combining readily with certain acids, particu- 

 larly nitric and oxalic. It also forms combi- 

 nations with certain salts, such as the oxide 

 - of mercury, chloride of sodium, etc. It exists 

 in the economy in a state of watery solution, 

 with perhaps a small portion of it modified by the presence of chloride of sodium. 



Origin of Urea. There are two probable sources of urea in the economy, assuming 





