Chemical Science. 193 



(wo similar racemates exist, but both are crystalline, the second 

 occurring in acicular crystals, which in sunshine become as white as 

 milk. The same change happens to the corresponding gummy tar- 

 trate, in which also minute crystals may occasionally be seen. 



The racemate of lime is much less soluble than the tartrate ; both 

 have the same composition, and both contain 4 atoms of water in 

 combination. The tartrate contains 21.765 per cent, of lime, and 

 the dried racemate gave 21.775 per cent, of lime. A solution of 

 sulphate of lime in water is decomposed by the addition of a little 

 racemic acid ; and after twenty-four hours, most of the lime is found 

 precipitated as racemate of lime. Tartaric acid does not do this. 

 If two solutions be prepared in muriatic acid, a little diluted, one 

 of racemate of lime, and the other of tartrate and then each be 

 saturated with ammonia, the racemate quickly falls as a semi-crys- 

 talline, white, opaque powder ; but the tartrate does not, unless the 

 liquid be much concentrated. After some time, octohedral crystals 

 form upon the glass. This is a good method of distinguishing the 

 acid, when one* of them is in solution. A solution of racemate of 

 lime, evaporated spontaneously, yields crystallized racemic acid ; but 

 evaporated by heat, the muriatic acid is volatilized, and the addition 

 of water dissolves no racemate*. 



20. ON GALLIC ACID. (By M. Braconnot.) 



M. Braconnot had recommended the preparation of gallic acid by a 

 process in which the tannin present in the infusion of galls was 

 removed by gelatine ; but M. Berzelius thought that such gallic 

 acid was chemically combined with tannin, and that pure gallic acid 

 could be obtained only by sublimation. M. Braconnot has there- 

 fore made further experiments, and finds that the two substances 

 differ, and that, not from the presence of tannin in the unsublimed 

 acid. The latter he calls pure gallic acid, and the other pyrogallic 

 acid. 



When very white gallic acid, giving no indication of tannin to 

 gelatine, was moderately heated, it became a brown liquid, which 

 crystallized on cooling, and which, dissolved in water, contained still 

 gallic acid and a brown substance which precipitated gelatine abun- 

 dantly. Thirty parts of dry white gallic acid, being subjected to a 

 higher heat, gave only 3J parts of sublimed gallic acid : though 

 very white, its solution precipitated gelatine. The residue, when 

 dissolved, gave a brown liquor, which became much deeper with 

 persulphate of iron, and blue-black with protosulphate (these being 

 characters of pyrogallic acid, and not of gallic acid) ; and it also 

 abundantly precipitated gelatine. Hence, heat appears to rearrange 

 the elements of gallic acid, so as to produce a peculiar variety of 

 tannin and pyro-gallic acid. 



Pyrogallic acid reddens litmus paper, though less than gallic acid ; 



* Aonales de Chiinie, xlvi., 128. 

 VOL. II. Aua. 1831. O 



