60 REPORT — 1839. 



older yeast, but the difference of size is not considerable. The re- 

 searches of Schulz, Cagniard de la Tour, and Schwann, appear to 

 show that the vinous fermentation, and the putrefaction of animal 

 matters — processes which have been hitherto considered as belong- 

 ing entirely to the domain of chemical affinity — are essentially the re- 

 sults of an organic development of living beings. 



Dr. Ure described at length the experimental processes by which 

 this position appears to be established. 



On the theory of the formation of White Lead. By Mr. Benson. 



The author, after describing at length the ancient and modern pro- 

 cesses for preparing white lead, proposed some new views of the 

 operation. The carbonate of lead formed by the common process is 

 anhydrous, amorphous, and contains one proportional each of carbonic 

 acid, oxygen, and lead. Now, as litharge is a protoxide of lead, it has 

 been thought, that in order to effect its conversion into white lead, 

 nothing more was requisite than to combine it with a due proportion 

 of carbonic acid ; and from this mistake a variety of fallacious pro- 

 cesses have been projected. The processes alluded to are founded 

 upon bringing the litharge into solution as a basic salt, and then pre- 

 cipitating it as a carbonate by the injection of carbonic acid. Painters 

 maintained, that this precipitate was not white lead. Chemists, find- 

 ing, by analysis, the correct proportions of protoxide of lead and car- 

 bonic acid, attributed the opinion of painters to prejudice ; but, by 

 microscopic observations. Dr. Ure has ascertained, that the carbonate 

 obtained by precipitation is semi-crystalline, and to a certain degree 

 transparent. The difference between white lead and precipitated car- 

 bonate may be illustrated by comparing them to pulverized chalk and 

 powdered marble ; both are carbonates, but the one is crystalline, the 

 other is not, and one is, consequently, less opaque than the other ; and 

 this difference is, of course, more appreciable, when the powders are 

 diffused thx-ough highly refracting media, such as oil. There is one 

 mode by which this difficulty may be avoided. The rationale of both 

 the processes, of that which produces the crystalline, and of that 

 which produces the amorphous carbonate, is the same. In both, the 

 lead is converted into basic acetate — in both, the salt is decomposed 

 by carbonic acid, but in the former the process is modified by the 

 pressure of water. In the one, the carbonate has been deposited from 

 a solution — in the other, the particles, never having departed from the 

 solid state, have not been at liberty to arrange themselves symmetri- 

 cally. In order, therefore, to produce amorphous carbonate, or white 

 lead, from litharge, it became necessary to present to the oxide of lead 

 a quantity of acetic acid so minute, that an insoluble basic salt should 

 be formed, with a quantity of moisture merely sufficient to determine 

 the action of the carbonic acid. The process would then resemble, in 

 all respects, the ordinary one, except that in the one the lead has been 

 previously converted into oxide, in the other, the formation of the 



