BULLETIN OF THE BUSSEY INSTITUTION. 101 
moved by way of decantation. The crystals are washed, by decan- 
tation, with a small quantity of cold water, and dissolved in cold 
water to a deep blue solution, the fitness of which for precipitating 
mannite was tested as follows: One tenth of a gram of pure man- 
nite, dissolved in 5 c.c. of water, was treated with a small quan- 
tity of the ammoniacal copper solution, the light blue precipitate 
was collected upon a filter, washed with cold water, and then sus- 
pended in warm water through which a current of sulphuretted 
hydrogen was passed. The black colloidal precipitate of copper 
sulphide was not readily filtered until after it had been boiled to 
make it cohere. ‘The filtrate from the copper sulphide was evapo- 
rated to a small bulk and left to cool slowly in a watch glass. 
After standing over night crystals of mannite were plainly visible. 
On being weighed they were found to amount to 14% of the 
mannite taken. In a duplicate trial the mannite crystals were. 
treated with ferrous sulphate and hydrogen dioxide, and finally 
with phenylhydrazin acetate. Mannose-hydrazone balls were 
obtained. 
Efforts that were made to treat the mannite-copper precipitate 
directly with hydrogen dioxide, that is to say, to omit the use of 
sulphuretted hydrogen altogether, failed. Thus, when 0.1 gram 
of mannite in aqueous solution was treated with the ammoniacal 
copper sulphate, and the washed precipitate, suspended in water, 
was mixed directly with a little ferrous sulphate and some hydro- 
gen dioxide, and subsequently treated in the usual way, no indi- 
cations of mannose hydrazone were obtained. In another trial a 
solution that contained 0.4 grm. of mannite was treated with the 
ammoniacal copper sulphate, and the washed precipitate was dis- 
solved in a small quantity of diluted hydrochloric acid (1:4) 
before adding the ferrous sulphate and hydrogen dioxide. No 
mannose hydrazone was obtained, and no better results were 
got in a third trial, where the mannite-copper precipitate was 
dissolved in dilute sulphuric acid before applying the hydrogen 
dioxide. 
Method of Percolation. 
In order to avoid the risk that some mannite might be formed 
by way of fermentation when plants supposed to contain it are 
leached with water, it was deemed advisable to percolate the mat- 
ters under examination with water maintained constantly at the 
