314 



Dr. Marcet on a Colloid Acid. 



[June 16, 



do not exist. The very abnormal conditions under which some of these 

 so-called organisms are found, would render it doubtful whether Bac- 

 teriums, Vibrios, &c., ought to be considered as independent organisms in 

 any higher sense than are white blood- corpuscles, pollen-grains, mucus- 

 corpuscles, or spermatozoa. 



VII. " On a Colloid Acid, a Normal Constituent of Human Urine. 

 By William Makcet, M.D., F.R.S. Received May 28, 1864. 



(Abstract.) 



The object of the present communication is to describe the mode of ex- 

 traction and the properties of an acid of a colloid nature which is always 

 present in healthy human urine, and appears destined to become of great 

 importance in Physiological Chemistry. 



With the view of separating this acid from the urinary secretion, the 

 fluid is mixed with animal charcoal, concentrated, and filtered, and the fil- 

 trate, after precipitation with baryta-water, is dialyzed for about twenty-four 

 hours. The dialyzed liquid, after subsequent filtration and concentration, is 

 mixed with basic acetate of lead, which precipitates the colloid acid as an 

 insoluble lead-salt, along with a little hydrochloric acid and other impuri- 

 ties. The precipitate should be thoroughly washed, decomposed with sul- 

 phuretted hydrogen, and again treated with animal charcoal. When the 

 acid is required in a pure state, the hydrochloric acid present is removed 

 with carbonate of silver, the excess of the silver precipitated with sulphu- 

 retted hydrogen, and, after boiling to evolve this last substance, basic acetate 

 of lead is again added. The lead-salt perfectly washed may be considered 

 pure, and the pure acid can be obtained from it by decomposition wiih 

 sulphuretted hydrogen. 



The acid is very slow to decompose when exposed to the air. It may 

 be considered to undergo no loss or decomposition by being boiled, as • 

 shown by direct experiment. After concentration by heat, its colour darkens 

 and it becomes syrupy, possessing a sharp acid taste, with a slight acrid 

 and astringent after-taste ; the taste is perceptible in the solution when 

 very dilute : no crystals of the acid could be obtained in the syrup. Dried 

 at a temperature under 212°F., the acid has the appearance of a transpa- 

 rent varnish ; it is very hygroscopic, and dissolves readily in water, though 

 not apparently in alcohol (sp. gr. "820) or in ether. When burnt, the col- 

 loid acid chars, emitting a pungent and irritating smell, and after complete 

 combustion, nothing but the minutest trace of inorganic residue remains. 

 Although strictly a colloid, this acid in the free state passes through a 

 dialyzer, but not so readily as a crystalloid. When under the form of a 

 compound, its property of dialyzing appears much diminished. I could 

 not find that it exerted any action on polarized light*. 



* This acid does not precipitate egg-albumen. It precipitates casein, but an excess 

 does not appear to redissolve the precipitate as in the case of acetic acid. 



