of a newly 'discovered vegetable Acid. 439 



tall glass jar of small diameter. While still hot, a stream of 

 sulphuretted hydrogen is to be transmitted through it; and when 

 all the lead has been precipitated, the fluid is to be filtered off, 

 and boiled in an open basin, until the discharged vapour no longer 

 blacken paper impregnated with acetate of lead. 



The theory of the process is obvious. When acetate of lead 

 is added to the juice, nialate of lead and the combination of the 

 new acid with lead precipitate ; the latter is decomj)osed by 

 boiling water into a super- and a sub-salt.: the super-salt is held 

 in solution ; but as the liqnor cools, the neutral compound de- 

 posits itself in crystals, and the first wasliings contain most free 

 acid. Vv'hen boiling water is no longer able to overcome the 

 attraction of the latter portions of acid to oxide of lead, no more 

 crystals can be formed. We then apply the Gtronger power of 

 sulphuric acid, we obtain the free vegetable acid, and proceed 

 as at first. When all the crystals are collected, such a quantity 

 of sulphuric acid is added as will be nearly suiilicieut to decom- 

 pose them : this is so done in order completely to exclude the 

 sulphuric acid, which without this precaution would be exceed- 

 ingly difficult to effect. The undecomposed portion of the cry- 

 stals dissolves in the vegetable acid newly extricated: but if in 

 the boiling the fluid were not continually stirred, a mass would 

 be formed in the bottom so hard as to resist decomposition. If 

 the li(juor after filtering were allowed to cool, the neutral salt 

 would crystallize ; it must therefore be used hot. The stream 

 of sulphuretted hydrogen passing through so high a column of 

 fluid soon separates the lead, while the piu'e vegetable acid is 

 liberated, contaminated indeed with a little sulphuretted hydro- 

 gen. This gas does not disappear completely by boiling, for the 

 acid retains the odour, be it ever so long boiled ; exposure for 

 a few davs in an open vessel dissipates it completely. 



In preparing this acid, it is not necessary, as it is in the pro- 

 cess for malic acid, to saturate the juice of the berries with potash, 

 at the commencement of the process; for, of the two compounds 

 formed after the mixture with acetate of lead, the malate dis- 

 solves in the evolved acetic acid in preference to the other. The 

 colouring matter, which adheres obstinately to the malate of 

 lead, is very apt, when extricated during the washing with boiling 

 water, to tinge the otherwise perfectly colourless crystals, which 

 form as the liquor cools. This is a great inconvenience, for the 

 colouring matter cannot be washed away, even by cold water, with- 

 out decomposing a (juantity of the salt: hence the only remedy 

 is, to reject all the crystals formed in the coloured washings, and 

 to reserve those only that are of a pure whiteness. The crystals 

 are of so delicate a fabric that they umst be separated by tlie 

 filter. When dried on paper, by exposure to air, they form a 



E e 4 white 



