on the magnetising influence qftlie Violet rays. S4S 
when acting upon polarised light, has been pointed out in the 
Philosophical Transactions for 1816. This analogy, which we 
shall have occasion to explain at some length in a subsequent 
paper, is so complete, that there are few phenomena in magne- 
tism which have not their counterpart in the action of crystallis- 
ed glass and regular crystals upon polarised light. 
Art. IV. — Examination qf some Compounds which depend up- 
on very weak Affinities. By Jacob Berzelius, M. D. F. R. S. 
and Corresponding Member of the Institute of France. Com- 
municated by the Author. 
( Continued from page 75. J 
Examination of Magnesia Alba. — it is well known, 
that when this earth is precipitated ’ from a warm solution, by 
carbonate of potash, an effervescence takes place. The magne- 
sia alba, therefore, contains less carbonic acid than the potash 
abandons. Is it a subcarbonate ? And, if so, what proportion 
subsists between the acid and the base ? My attempts to resolve 
these questions led to a result very different from what I had 
at first anticipated. 
The analyses which we possess of this substance give very 
discordant results, as the following table will testify : 
Bergman. 
Kirwan. 
Butini, 
Klaproth. 
Magnesia, 45 
45 
40 
40 
Carbonic acid,.. 30 
34 
36 
33 
Water, 25 
21 
21 
27 
The great ease with which the quantity of caustic magnesia, 
remaining after the calcination of this salt, may be accurately 
determined, forbids us to suppose that any of the analyses are 
erroneous in that particular. But the quantities of carbonic 
acid and of water, may have been somewhat modified by the 
method followed for obtaining them. 
It is easy to see, that, according to none of these analyses, is 
the carbonic acid combined with twice as much oxygen, as in 
the ordinary carbonate. It is farther evident, that the first 
three analyses, in no respect agree with the laws of chemical 
proportion; whilst that of Klaproth makes the carbonic acid 
if ^ 1 ^=.^ 
