6 Dr. Thomas Thomson [July 



table blues to red, and the change is permanent. Various 

 opinions have been formed respecting the nature of this 

 acid. It was supposed at first to be the phosphoric. This 

 was the opinion of Proust, and of Fourcroy, and Vauque- 

 lin. Urine contains a minute quantity of phosphate of 

 lime, which may be precipitated in the state of a light 

 white sediment by caustic ammonia. Now as phosphate of 

 lime is insoluble in water, while a little of it is actually 

 held in solution in urine, it was not unreasonable to con- 

 clude that the salt in urine might be in the state of biphos- 

 phate of lime ; as in that state it is slightly soluble in 

 water, and has the property of changing vegetable blues to 

 red. But a very simple experiment is sufficient to show 

 that urine contains no biphosphate of lime. If we evaporate 

 a quantity of urine to dryness and ignite the dry residue, 

 the residual salts do not act on litmus paper. Hence it is 

 obvious that the free acid in urine must be volatile, since 

 it is dissipated by a red heat. 



Berzelius affirms that urine contains lactate of ammonia 

 and lactic acid; and to the presence of these bodies he as- 

 cribes the peculiar smell and colour of urine, as well as its 

 property of reddening vegetable blues. As I have never 

 been able to satisfy myself of the presence of these bodies 

 in urine by experiment, it is impossible to adopt Berzelius' 

 opinion respecting them. A brown coloured matter may 

 certainly be obtained from urine, but I have always got 

 it in such minute quantity that I was unable to determine 

 any thing positive respecting its nature. 



Thenard has substituted acetic acid for the lactic acid of 

 Berzelius, probably because Fourcroy and Vauquelin had 

 concluded from a set of experiments made many years ago, 

 that lactic acid is merely acetic contaminated by or com- 

 bined with a little animal matter. But this position was 

 by no means established by their experiments. In order to 

 be able to form some definite notion on the subject, I 

 mixed sulphuric acid with fresh urine till it tasted dis- 

 tinctly acid, and distilled over one-third 9f the mixture 

 from a retort by means of a gentle heat. The liquid which 

 came over was tasteless and had no perceptible smell ; it 

 slightly but evidently reddened litmus paper. It was mixed 

 with carbonate of soda till it became sensibly alkaline ; be- 



