16 BULLETIN OF THE BUSSEY INSTITUTION. 
though a beginner might perhaps do well to prepare a small 
quantity of mannose expressly by hydrolyzing date stones or the 
ivory nut, and comparing the reactions of this substance with 
those exhibited by a suspected material. 
To make assurance doubly sure, it is well to reerystallize from 
warm diluted alcohol the mannose-hydrazone that was first thrown 
down, and to view the new crop of crystals under the microscope. 
By so doing, the globules of mannose-hydrazone may be changed 
to the highly characteristic crystals with bevelled ends, and a 
cleaner field be obtained also for viewing the crystals. One con- 
stant trouble, for instance, in testing for mannose, even in mate- 
rials that are known to be unusually pure, is the formation of the 
above-mentioned fine amorphous yellow powder on the addition 
of the phenylhydrazin reagent. Since it might readily deceive 
the uninstructed or unaided eye, it is absolutely necessary to 
examine the precipitate with the microscope in order to be sure 
that globules or crystals of mannose-hydrazone are contained in 
it. It seems probable that some part of this yellow dust may be 
formed by the union of phenylhydrazin with an oxidation product 
of dextrose. It may not be connected in any way with the 
presence of mannose, for it forms readily in solutions that contain 
no mannose. Even aqueous solutions of pure dextrose give faint 
yellow precipitates on being tested with phenylhydrazin, as will 
be set forth directly. Happily, this yellow dust is soluble in 
glacial acetic acid, and it may be removed from the field of the 
microscope by applying a drop of this acid. 
For testing the various points above mentioned a quantity of a 
solution of tolerably pure mannose was prepared by hydrolyzing 
vegetable ivory with sulphuric acid, and numberless experiments 
were made by taking measured quantities of the standard solution 
thus obtained, diluting them with varying quantities of water and 
‘adding to definite volumes of these solutions stated numbers of 
drops of one or another of the phenylhydrazin reagents, taken in 
varying degrees of concentration and applied in such manner that 
sometimes the phenylhydrazin and sometimes the mannose should 
be in excess. The results of these trials showed that tolerably 
concentrated solutions are needed in order that satisfactory results 
may be obtained. If the solution of mannose is too dilute, or, in 
general, if too small a quantity either of mannose or of phenyl- 
