1864.] Dr. Marcet on a Colloid Acid of Urine. 3 



As the lead precipitate appears to be slightly soluble in water, it is diffi- 

 cult to ascertain precisely the period at which the washing may be con- 

 sidered sufficient. I usually continued pouring distilled Water into the 

 filter until sulphuretted hydrogen gave but a faint dark colour in the 

 filtrate. The decomposition of the insoluble compound by means of sul- 

 phuretted hydrogen is a slow process, and takes some hours before it is 

 complete ; and it will be advisable to leave the precipitate in contact with 

 sulphuretted hydrogen overnight : the excess of sulphuretted hydrogen 

 may be expelled by boiling, or by a stream of air, by which latter method 

 the coloration caused by heat is avoided. 



The Acid. The fluid prepared as stated above has a strong acid reaction; 

 exposed to the air for a fortnight, and even for a longer period, it is not 

 altered. When concentrated even by very brisk boiling, it may be con- 

 sidered as undergoing no loss and no decomposition, as shown by the fol- 

 lowing experiments : 75 cub. centims. of the acid, 5 cub. centims. of which 

 were neutralized by 24'3 cub. centims. of a normal potash solution, were 

 boiled down to 15 cub. centims., then diluted with water to 75 cub. cen- 

 tims., and tested with the normal potash solution ; 23'4 cub. centims. of 

 this solution were found necessary to neutralize it. Any decomposition 

 must have been very slight, as the difference between the volumes of the 

 potash solution added before and after the boiling was only of about 1 cub. 

 centim. 



As a further proof that the acid is not volatile, I distilled a sample of it 

 in a retort, in the free flame, carrying on the operation until nothing but a 

 dark semifluid mass remained in the retort. The distillate was just faintly 

 acid to the most delicate test paper. In most of my experiments the acid was 

 slightly coloured, and evolved a urinous smell when hot. Under the impres- 

 sion that this odoriferous substance might be a volatile acid, such as those 

 discovered by Stadeler *, I was led to examine very carefully the distillate 

 obtained as above. This fluid had a strong smell of urine and the faintest 

 acid reaction. After the fluid was rendered alkaline by baryta, the smell 

 was in no way diminished ; so that it could not be owing to an acid ; more- 

 over, considering that the colloid acid loses its colour, and apparently in a 

 great measure its odour, after agitation with animal charcoal, we may infer 

 that the odour of urine is owing to a very slight decomposition of the col- 

 loid acid which takes place under the influence of heat, and more especially 

 in the presence of free mineral acids. 



Returning to the solution of the colloid acid in water : after concentra- 

 tion by heat its colour darkens and it becomes syrupy, with a sharp acid 

 taste, and a slight acrid and astringent after-taste. This taste is percepti- 

 ble in the solution, even when very dilute. I could never obtain any 

 crystals in this syrup beyond those resulting from inorganic impurities. 

 When dried, the acid assumes the form of a transparent varnish, which, 

 by a temperature of 120 Cent., becomes much darkened. The dried 



* Annal. der Chemie und Pharm. vol. xcvii. p. 134. 



B 2 



