THE STUDY OP TARTARIC ACID. 
35 
is reduced, and water and carbonic acid extricated. If we 
now endeavor to dissolve the salt in water, a quantity of black 
ferruginous matter will subside, corresponding in amount to 
the portion of the salt decomposed. 
This easy decomposition of the tartrate of iron and potassa, 
renders it unsuitable to elucidate the question which we have 
proposed, but it gives us the key to a fact which is known to 
those who have managed this salt, and which has hitherto re- 
mained unexplained, namely: that when evaporated by a 
naked fire it often happens that the dried salt refuses to dis- 
solve in water. This results from the elevation of tempera- 
ture during the drying, being sufficient to produce a partial 
deoxidisement of the iron. 
This ready reduction of the oxide of iron in the tartrate of 
iron and potassa, appears the more remarkable to us in an- 
other experiment, by which we endeavored to produce a dou- 
ble tartrate, in which the oxygen should be in the same quan- 
tity in the potassa and in the oxide of iron, or even in a 
double quantity in this latter. For this purpose we kept at a 
boiling temperature, in two different mattrasses, the bitartrate 
of potassa, and the hydrated sesquioxide of iron in the quan- 
tities proper to produce these results. At first, the liquid 
was highly colored on taking up the iron, but all at once it 
became colorless, and at the same time a deposit, nearly co- 
lorless, appeared at the bottom of the vessel. This sediment 
was tartrate of iron; it resulted from the partial reduction of 
the sesquioxide of iron by the elements of the tartaric acid. 
We have produced the same result by boiling a mixture of 
cream of tartar and tartrate of iron and potassa, and even by 
boiling a solution of this latter salt, when perfectly pure. It 
may now be readily seen why we have recommended, in the 
preparation of this tartrate, that the temperature should be 
between 50° and 60° c, and how, with an excess of hydrate 
of the sesquioxide of iron, solutions might be obtained which 
were not saturated with the iron. This is probably what 
happened to Mr. Philips; it is what we have observed when 
this property was not yet known to us in our first experiments, 
