Prof. Pliicker on the Magnetic Induction of Crystals. 477 
tute for sulphuric acid, with bi-borate of soda, for boracic acid, with 
alumina and sesquioxide of iron. Of these bodies only boracic acid 
gave results similar to those obtained with silicic acid. The other 
substances all gave the equivalent numbers usually assigned to them, 
equally with the carbonates of potash and soda. 
Directing his attention then to determine whether the equivalent 
of silicic acid could be found in other volatile acids than the carbonic, 
the author relates some experiments made with the hydrates of 
potash and soda, but he explains that there are circumstances which 
render it much more difficult to obtain accordant numbers with 
these bodies than with the carbonates. 
Six experiments made with hydrate of potash gave as mean result 
_ the same number as that obtained with the carbonate, viz. 30°8. 
But with hydrate of soda the mean of three experiments gave the 
number 17°2 as the equivalent of silicic acid,—a result approaching 
that previously obtained with carbonate of lithia. 
A silicate of soda was formed by fusing together silica and carbo- 
nate of soda, in proportions indicated by the previous experiments— 
i.e. 23 silica to 54 carbonate of soda = 31 soda. The fused mass 
was crystallized. It was dissolved in water, and the solution evapo- 
rated in vacuo yielded a crystallized salt, which contained about 
5 per cent. of carbonate of soda; when this, calculated as the or- 
dinary 10-hydrated salt, was subtracted, the silicate agreed nearly 
with the formula NaO.SiO,+7HO. A crystallized salt of like 
composition was obtained when hydrated silica was dissolved in a 
solution of caustic soda, the silica and soda being in the same 
proportions, i. e. 31:23. In these experiments it appears, there- 
fore, that in the process of solution and crystallization a portion of 
soda is extruded. When exposed to a temperature of about 300° F., 
nearly all the water was driven off from these salts, less than 1 per 
cent. remaining. 
The fused silicate of lithia in like manner, when treated with water, 
appeared to split up into different compounds. 
After guarding himself from drawing any decided inference from 
the experiments recorded, the author concludes by observing that at 
present he can see no alternative but to admit of more than one 
equivalent for silicic acid (that is to say of more than one acid), the 
value of which is determined by circumstances,—such as the presence 
of water and the nature of the base to which it is presented. The 
existence of such different silicic acids has been already suggested by 
chemists on different grounds, particularly by Ebelman and Laurent, 
and lately by M. Fremy. 
April 23.—The Lord Wrottesley, President, in the Chair. 
The following communication was read :— 
“On the Magnetic Induction of Crystals.” By Professor Julius 
Pliicker of Bonn, For. Memb. R.S., Hon. M.R.I. &e. 
The author commences by referring to his discovery of the peculiar 
action of magnets on crystalline bodies, and to the researches to 
