| On the Nature and Combinations 
ceive the difficulty and expense of it: and I have found that the 
acid obtained is variable in its nature. In one case I obtained 
an acid, which, when mixed with solution of acetate of lead, did 
not at first produce any effect, but at length slowly deposited a 
precipitate. The heating of another portion of the acid with 
carbonate of lime, produced a separation of a black powder, 
which possessed the properties of charcoal. There were also 
many other peculiarities; and the combined effect of all was to 
convince me, that great differences exist between the acid ob- 
tained in this manner, and that obtained by other processes. 
The experiments of Vauquelin satisfied him that the acid which 
is combined with lime in the Senpervivum Tectorum, is the true 
malic: and all my trials convince me, that it does not contain 
even the least quantity of the sorbic. Since then, by the means 
generally employed, we do not obtain malic acid, the only alter- 
native is to adopt the hitherto neglected process of Vauquelin ; 
and it will be found that his process affords the acid with greater 
ease, and in much greater purity, than any other. The method 
of detaching the acid from the malate of lead by sulphuric acid 
is, as we have seen, difficult; and the criterion of the taste is 
liable to this fallacy, that as the sourness increases, the sweet- 
ness decreases. There will, on this account, be a period when 
the latter will disguise the former, and yet the lead will be still 
present. I would therefore suggest the substitution of sulphu- 
retted hydrogen in place of sulphuric acid. 
If it were required to obtain malic acid excéedingly pure, and 
still more divested of vegetable matter, the following process 
may be adopted. 
The juice of Sempervivum Tectorum is to be evaporated to 
two-thirds, and, after standing some hours, it is to be filtered, 
and mixed with an equal quantity of alcohol. ‘The coagulum 
is to he separated by the filter, edulcorated with fresh portions 
-of alcohol, and dried in the air, lest any adhering alcohol should 
impede its subsequent solution. The mass is then to be dis- 
solved in water, mixed with solution of acetate of lead, and 
the precipitate collected on a filter. After being well edulco- 
rated from any superfluous acetate of lead, the precipitate is to 
be boiled for fifteen minutes with a little less of dilute sulphuric 
acid than is sufficient to saturate the oxide of lead: and for this 
part of the process, the criterion of sweetness will very well 
answer the propose. The whole is to be set aside for some days, 
and, during this period, a small quantity of sulphate of lead 
which the malie acid held dissolved, will be deposited. The 
liquor is now to be filtered, and in order to separate the last 
portions of lead, a stream of sulphuretted hydrogen is to be 
transmitted through it: the black precipitate is to be filtered 
off, 
