290 
PREPARATION OP FORMIC ACID. 
and partly, we think, because more of the sugar is carried 
to its highest stage of oxidation in the forms of carbonic 
acid and water. We have therefore been led to another, 
and we believe, better mode of operating, of which the fol- 
lowing details will serve as an example. 
Introducing into a retort, capable of holding about one 
quart, 800 grains of bichromate of potassa and 10 cubic 
inches of water, we gently heat the mixture so as to dis- 
solve the larger part of the bichromate. We then add 300 
grains of powdered white sugar, and adjusting to the tubu- 
lure a perforated cork and pipette with gum-elastic bag for 
the gradual introduction of sulphuric acid, we slowly inject 
about 1 cubic inch of the latter upon the mixture. By 
regulating the addition of the acid and occasionally inter- 
mitting the slender stream, the violent reaction which en- 
sues is prevented from occasioning any very great intu- 
mescence. During this stage of the operation, upwards of 2 
cubic inches of a clear but feebly acid liquid passes over 
into the receiver. When the action has in a good measure 
subsided, we add 5 cubic inches more of water, and apply 
a gentle lamp heat, continuing the addition of the acid, by 
allowing it simply to drop from the pipette, until another 
cubic inch has been introduced. The liquid which now 
passes over is much stronger in formic acid than in the pre- 
ceding stage, and the distillation may, without impairing 
the purity of the product, be continued until about 7 cubic 
inches have been received. By urging it much beyond this 
point sulphurous acid will be evolved. 
One hundred grains of the liquid thus obtained is capable 
of saturating about seven grains of dry carbonate of soda. 
Its purity is such as to fit it for immediate use in illustrating 
the striking reactions of formic acid and the formiates. 
Thus— 
1. On adding a small portion of it to a solution of nitrate 
of silver previously curdled by ammonia, and applying heat, 
the silver is promptly reduced with a lively effervescence of 
carbonic acid. 
