1817.] Juice of the Stems of Rhubarb. 105 
present in the resulting liquor, and might be obtained by evapora- 
tion in a crystalline form. Other salts might also have been present. 
Mr. Henderson no doubt satisfied himself that these acid crystals 
could not have been those which he considered as the rheumic acid. 
I examined the juice of the stalks of rhubarb within the last 
month. The sorbic acid had disappeared on account of the lateness 
of the season; the malic was present, but I could find no other. 1 
do not, however, hence presume to offer any thing opposing the 
statements of Mr. Henderson. The rheumic acid, like the sorbic, 
might have disappeared. It is apparent that Mr. Henderson ob- 
tained malate of lime in his process ; for he found that even when 
the juice contained much more chalk than it could dissolve, the 
supernatant liquor still reddened litmus. And taking every thing 
into consideration, it is plain that as the compounds, from the pro- 
perties of which the peculiar nature of the new acid has been in- 
ferred, were formed by means of a mixture of several acids, they 
cannot be looked on as decisive in the question. 
Mr. Henderson conceives that the new acid exists in the plant in 
combination with ammonia. But it should be considered that ex- 
tractive matter and lime were presented to each other, and that by 
the mutual action of these substances ammonia is always produced. 
These observations I have been induced to make, as the ingenious 
and candid author of the paper on which I comment acknowledges 
imperfections in his process, and expresses a wish that some other 
person might assist him in the examination of this subject. The 
preceding might probably be of some advantage. 
Artic Le III. 
On the Composition of the Topax; the Separation of Silica and 
Oxide of Tantalum; und some further Experiments on the Com- 
position of Organic Bodies. By Jacob Berzelius, M.D. F.R.S. 
Professor of Chemistry at Stockholm. 
(To Dr. Thomson.) 
SIR, Stockholm, Dec. 2, 1816. 
In the number of the Annals of Philosophy for October a very 
celebrated mineralogist, Mr. Gregor, has communicated some ex- 
periments on the composition of the topaz. He has drawn asa 
conclusion that this stone contains potash. I have made many ex- 
periments, as you know, on this mineral; and I consider myself as 
having ascertained its true chemical constitution. At the same 
time I did not try to discover alkali in it; because in my analyses of 
it, though often repeated, I never experienced any other loss than 
that which is almost unavoidable in experiments of that nature. 
After reading Mr. Gregor’s letter, I thought it necessary to make a 
