which depend on WeaJc Ajffinities, 6T 
must be composed either of two double salts, which have one of 
their bases in common, or of a double salt, combined with one 
or more molecules of a simple salt. In more general terms, it 
must be composed, either of two compound bodies of the third 
order, or of one compound body of the third, and another of 
the second order. But at present we know not the number to 
which the orders of compound atoms may amount, or how many 
atoms of the second order may meet together in the same com- 
bination. 
Another circumstance about which we have obtained no cer- 
tainty, occurs in that species of combinations, into which one of 
the ingredients enters only in very small quantity ; such, for 
example, as plumbago, in which one molecule of iron must be 
combined with about 200 molecules of carbon. It is very diffi- 
cult, in such cases, to distinguish between a foreign mixture and 
an actual combination. In analytical experiments upon mine- 
rals, those bodies which appear only in very small quantities, are 
generally considered as accidental. And although this principle 
may sometimes lead to erroneous results, it is advisable to adopt 
it in cases where its inaccuracy has not a certain degree of pro- 
bability ; because without some such help, our researches would 
be enveloped in endless difficulties. After obtaining a general 
acquaintance with the subject, it will be easier to rectify the 
mistakes which may have happened in particular cases. 
Eocamination of a Double Carbonate^ having a Base (^Pot- 
ash and Magnesia. — Carbonic acid bears a resemblance to silica, 
oxide of tantalum and oxide of titanium, in the weakness of 
its affinities, and in having a greater tendency, than the strong 
acids, to form compounds analogous to those of silica. The 
double salt which we are now to describe, is a proof of this.— 
A solution of bi-carbonate of potash was mixed (slightly in excess) 
with a solution of the muriate of magnesia. No precipitate was 
formed ; but some days afterwards,, a salt had arranged itself in 
crystalline groups, upon the bottom and sides of the vessel. 
When first separated from the liquid, this salt had no taste, 
but a few moments afterwards it became alkaline. In pure 
water it appeared insoluble at first ; but some time after it fell 
into powder, and the water dissolved carbonate of potash from 
it, leaving carbonate of magnesia undissolved. When heated to 
