160 BASIC BODIES. 



examination. A good crystallometric determination yields, how- 

 ever, the same certainty as an elementary analysis which, in these 

 cases, would never, or extremely seldom, be possible. 



Formerly the presence of small quantities of urea was supposed 

 to be established when chloride of sodium crystallised in the octo- 

 hedralform; but independently of the circumstance that other sub- 

 stances besides urea may induce a similar action on the form of the 

 crystals of this salt, it must be borne in mind that chloride of 

 sodium, when we trace the formation of its crystals under the 

 microscope, presents itself in combinations of the regular system, 

 with a complexity varying with the minuteness of the crystals. 

 This occurs when we allow pure chloride of sodium to crystallise ; 

 and it is still more the case when organic matters are mixed with 

 the solution. I am acquainted with no other substance of the 

 regular system which presents such uncommon crystals under the 

 microscope as chloride of sodium. We need only expose the alco- 

 holic extract of any animal fluid to spontaneous evaporation, in 

 order to recognise with the naked eye, in the greater crystals, the 

 combinations which we have perceived on examining the crystalli- 

 sation of a solution of pure salt under the microscope. 



In order to determine the amount of urea in urine, most ana- 

 lysts have followed the method proposed by Mitscherlich,* and have 

 availed themselves of the insolubility of the nitrate. There are 

 several causes of error in this method which cannot be altogether 

 avoided, but with due care may be made very inconsiderable. 

 They chiefly consist in the imperfect insolubility of this salt, 

 and on the adherence of the so-called extractive matters to it ; if, 

 however, we use an excess of nitric acid for the purpose of sepa- 

 rating the urea, cool the fluid artificially, filter after some time, 

 rinse the salt with cold nitric acid, and, after it has been submitted 

 to pressure, dry it at a temperature not exceeding 110, we shall not 

 have so great a loss of urea as Heintzf maintains must always occur 

 in adopting this method ; but in relation to accuracy, the results fall 

 far short of those obtained in the determination of mineral sub- 

 stances. The idea occurred almost simultaneously to RagskyJ and 

 Heintz that the urea in urine might be determined quantitatively 

 by its decomposition by sulphuric acid. Both investigators have 

 satisfied themselves that the so-called extractive matters of the 



* Pogg. Ann. Bd. 31, S. 303. 



t Ibid. Bd. 66, S. 114-160. 



t Ann. d. Ch. u. Pharm. Bd. 56, S. 29-34. 



Pogg. Ann. Bd. 68, 8. 393-410. 



