1864.] 



Dr. Marcet on a Colloid Acid* 



315 



Composition of the Colloid Acid. 



The acid was found to consist only of carbon, hydrogen, and oxygen. I 

 have not yet succeeded in establishing its ultimate quantitative composition, 

 but it appears to be very poor in hydrogen and rich in carbon. The 

 atomic weight of the substance was found by the analysis of its insoluble 

 lead-salt, and of its baryta-salt. I determined the lead in the lead com- 

 pounds from six different samples of urine ; the average in 100 parts was — • 



Oxide of lead , 66-3 



Acid.. 33*7 



100-0 



The analysis of the baryta compound yielded in 100 parts — 



Baryta 72*2 



Acid 27-8 



100-0 



Corresponding to the atomic weights 



Oxide of lead ' . . . . 111-5 



for the lead compound 



Acid 56-7 



168-2 



for the baryta compound \ '^^ ^ 



^ I Acid 29-5 



106-0 



It is therefore very obvious that the acid forms two salts, an acid and a 



neutral salt; we shall adopt the number 28-35 ^or^^^ for the atomic 



weight of the new acid. The fact of there existing two different com- 

 pounds of the acid, explains many chemical phenomena exhibited by this 

 substance and its salts. 



Compounds of the Colloid Acid of TJriyie, 

 The neutral salts are all soluble. 



Lead-Salts. — The colloid acid forms two lead-salt — sone which is insolu- 

 ble in water, and contains two equivalents of acid, and one which is so- 

 luble in water, and evidently contains one equivalent of acid. 



The insoluble compound is obtained by adding basic acetate of lead to 

 an aqueous solution of the acid or of its neutral salts. An excess of the 

 basic acetate redissolves the precipitate, which reappears on the addition 

 of dilute nitric acid, to be finally redissolved in an excess of the mineral 

 acid. The whole of the colloid acid is not, however, precipitated by basic 

 acetate of lead, principally on account of the formation of a certain quantity 

 of neutral acetate of lead, which I found to have the property of dissolving 

 the insoluble colloid lead-salt. On boiling a mixture of the insoluble lead 



