398 
Tin • 
immersed in it, is that of the lowest degree of oxydation, 
which is obtained either by keeping some undissolved tin 
in the solution or (Exp. 10 and 11) by introducing iron. 
The great difficulty which occurred to the author of these 
experiments was to account for the precipitation of the tin 
when there did not appear to be any copper dissolved du- 
ring the process, for in all other metallic precipitations (as 
that of copper on iron for example) the separation of one 
metal from the solution is attended with the solution of a 
corresponding quantity of the other metal. 
The difficulty is increased too by the fact that when 
tin is immersed in a solution of copper the copper is pre- 
cipitated, and a portion of the tin is dissolved, so that the 
direct affinity of acids for tin is greater than for copper. 
The hypothesis given by Professor Gadolin to explain 
this is highly ingenious. He observes that as a very strong 
affinity exists between tin and copper, this affinity ope- 
rates on the metallic part of the solution of the tin imme- 
diately in contact with the copper, and causes its separa- 
tion in the metallic form, whilst the remainder of the tin, 
which is the greatest portion, undergoes a greater degree 
of calcination (i. e. oxygenation) and hence too the solu- 
tion after its utmost action on copper still retains a quan- 
tity of tin. In other words, the dissolved tin divides itself 
into two portions, one of which returns to the metallic 
state, and adheres to the copper, whilst the whole of the 
oxygen condenses in the other portion, which remains 
dissolved. 
To shew that a corresponding solution of the precipi- 
tant {\* e. the copper) is not necessary to the separation of 
the precipitate , M. Gadolin boiled some fine gold with 
tin and tartar, and after a short time the gold was com- 
pletely covered with a coating of reduced tin, and it can- 
not be supposed that any of the gold should have been dis- 
solved by the tartar. This experiment would have been 
