1864.] Dr. Marcet on a Colloid Acid of Urine. 7 



pitate ; and on mixing some of the precipitate with a solution of neutral 

 acetate of lead, and boiling, I observed a solution of the colloid compound 

 to take place. This experiment, repeated several times with different 

 samples of the insoluble lead-compound of the colloid acid, yielded invari- 

 ably the same result : although it happened in most cases that the preci- 

 pitate was not entirely dissolved, it was very obvious that the greater 

 portion of it had disappeared. The phenomenon in question is very 

 interesting, not only because it accounts partly for the fact that basic 

 acetate of lead does not precipitate the whole of the colloid acid of urine, 

 but also because it affords additional proof of the insoluble lead-compound 

 being an acid lead-salt of the colloid acid. A piece of wet blue test-paper 

 is reddened when held over the opening of a test-tube in which the mixture 

 of the precipitate and neutral acetate of lead is being boiled, showing that 

 acetic acid is given off. This would not happen unless there was an acid 

 present to displace the acetic acid from the neutral acetate of lead ; and 

 this acid must be the acid lead-salt. There is now no difficulty in ac- 

 counting for the solution of the acid lead-salt in neutral acetate of lead : 

 one equivalent of the colloid acid combines with one equivalent of lead of 

 the neutral acetate, two equivalents of neutral lead-salt of the colloid acid 

 being formed, PbO . 2 (acid) + PbO C 4 H 3 O 3 =2 (PbO.acid) + C 4 H 3 O 3 , 

 while free acetic acid is evolved. On incinerating the acid lead-salt of the 

 colloid acid, it chars and leaves a residue consisting of metallic lead. 



By boiling the free acid with a small quantity of hydrated oxide of lead, 

 I have often been surprised at the small proportion of lead dissolved, 

 which is apparently owing to the lead being entirely transformed into the 

 insoluble acid salt. After being boiled with a larger quantity of the oxide 

 and filtered, still acid, a precipitate takes place in the fluid on cooling. 

 This precipitate must be a lead-salt of the acid. I have not determined 

 its quantitative composition. With an excess of hydrated oxide of lead, 

 the colloid acid forms a compound in a great measure insoluble in hot 

 water. After boiling the colloid acid with hydrated oxide of lead, it was 

 observed that a yellowish- green crystalline deposit had formed in the 

 capsule ; these crystals, on being burnt, appeared to contain but traces of 

 organic matter. I have not yet been able to determine their nature ; but it 

 is difficult to believe them to be a compound of the organic acid with lead, 

 this acid being strictly colloid in all its other properties. When a solution 

 of the salts of the colloid acid is boiled with hydrated oxide of lead, a 

 portion only of the acid is precipitated ; so that this method is not available 

 for the extraction of the acid from urine. 



When the colloid acid is boiled with peroxide of lead, some lead is dis- 

 solved, and the solution becomes neutral, or but very faintly acid. The 

 acid dissolves silver from the carbonate, but it is not possible to neutralize 

 it completely thereby. When boiled with black oxide of copper, some 

 copper is dissolved by the colloid acid. 



The Baryta-Salt. When the colloid acid of urine is boiled with carbo- 



