6 Dr. Marcet on a Colloid Acid of Urine. [1864. 



If we now compare the atomic weight of the acid in the baryta-com- 

 pound (29(5) with that in the lead-compound (56'7), it will be readily seen 

 that the relative proportion of these two numbers being very nearly one to 

 two, the lead-compound contains two equivalents of colloid acid (2 x 28'3), 

 and the baryta-salt one equivalent of the acid (28*3) ; the insoluble lead- 

 compound is therefore an acid salt of the colloid acid. I shall propose 

 the number 28'3 as the atomic weight of the new acid. "We are now 

 able to explain why the insoluble lead-salt of the colloid acid is soluble in 

 an alkaline fluid such as potash. Of the two equivalents of colloid acid, 

 one combines with oxide of lead, and the other with potash, forming two 

 soluble neutral salts, 



PbO . 2 acid+KO=PbO . acid+KO . acid. 



Compounds of the Colloid Acid of Urine. 



Lead-salts. Basic acetate of lead added to the free acid, or solutions of 

 its neutral salts, gives rise to a white precipitate. When a glass rod 

 moistened with a solution of basic acetate of lead is immersed into a mode- 

 rately strong solution of the free acid, a precipitate forms, which disappears 

 on agitating the fluid ; this can be repeated several times before the pre- 

 cipitate becomes permanent. When the fluid no longer turns clear on 

 agitation, the application of heat will dissolve the precipitate, but on the 

 further addition of the precipitant, the hot liquid will soon remain turbid. 

 An excess of basic acetate of lead redissolves the precipitate ; this re- 

 solution appears to take place more readily when an excess of the precipi- 

 tant is added to neutral salts of the colloid acid, than when added to the 

 free acid. 



After mixing basic acetate of lead with urine, treated as described above, 

 and filtering, I observed that the filtrate still contained much organic 

 matter, although the further addition of the reagent caused no turbidity. 

 I at first naturally thought that this was owing to the presence of another 

 colloid substance in urine ; but my surprise was very great when I found 

 that the pure acid obtained by decomposing its insoluble lead-compound 

 could be but partly re-preeipitated by means of basic acetate of lead, a 

 comparatively large portion of organic matter remaining dissolved, as shown 

 by evaporating a few drops of the fluid and incinerating the residue, which 

 charred and burnt away, leaving a little oxide of lead on the spatula. 

 Having previously ascertained that the pure basic lead-salt was nearly 

 perfectly insoluble in water, I could not for some time explain the phe- 

 nomenon. On reflecting upon the fact, it occurred to me that, as the liquor 

 in which the acid had been precipitated appeared to contain some neutral 

 acetate of lead*, possibly this salt had the power of dissolving the preci- 



* The formation of this compound might be considered to have taken place as shown 

 in the following equation : 

 PbO] 



PbO LC r H,0,+4 acid*=2 (PbO, 2 acid)-fPbO, C, H,0,. 

 PbOj 



