44 BULLETIN OF THE BUSSEY INSTITUTION. 
instances, that by the reaction of silver acetate on the metallic 
lead employed to remove the excess of silver from the solution 
enough lead acetate may be formed to prevent the separation of the 
mannose-lead precipitate which is known to be soluble in an excess 
of this reagent. It was found, indeed, on trial that a small 
sample of the gelatinous mannose-lead precipitate gradually dis- 
solved on being treated with a solution of normal lead acetate; 
and that, when a small quantity of a solution of sodium acetate 
was,mixed with a weak solution of mannose, no precipitate formed 
therein on the addition of a drop of basic lead acetate, although 
in the absence of the sodium acetate a precipitate fell readily. 
In addition to these difficulties, it is to be remembered that solu- 
tions of gums of various kinds, as well as of pectin and vegetable 
mucilage, are readily precipitated by basic lead acetate, even from 
highly dilute solutions. According to Staedeler* dextrin also 
may yleld a gelatinous precipitate when a concentrated solution 
of it is mixed with basic lead acetate. 
It needs to be said, however, that instances may occur where, 
although the lead acetate would be of no use as a distinguishing 
test for mannose, it might serve fairly well for separating this 
sugar from an aqueous solution that contained it naturally beside 
various other soluble vegetable matters. In this event, the man- 
nose might be recovered from the solution in a somewhat more 
concentrated form than it was before by precipitating it, together 
with gums, etc., by means of basic lead acetate, washing the 
precipitate with cold water, and decomposing it either with sul- 
phuretted hydrogen — in presence of acetic acid —or with dilute 
sulphuric or hydrochloric acid, or by means of carbonic acid gas. 
In experiments made to test this method of procedure, a small 
part of the mannose-lead was put aside, while a slight excess of 
dilute sulphuric or hydrochloric acid was added to the remainder. 
After time enough had been allowed for the decomposing action 
of the acid, the reserved portion of the precipitate was stirred 
in to neutralize the excess of acid. The mannose solution was 
separated by filtration, evaporated to a small bulk, taken up with 
water and again filtered, if need were, before testing with phenyl- 
hydrazin. Or, if dilute sulphuric acid is used, a slight excess of 
it may be added to the whole of the precipitate, and the mixture 
* Annalen der Chemie und Pharmacie, 1859, 111. 26 note. 
