BULLETIN OF THE BUSSEY INSTITUTION. 43 
sulphate, the second and third precipitates exhibited the charac- 
teristic gelatinous appearance of the mannose-lead compound even 
more emphatically than the first precipitate did. But since the 
mannose-lead precipitate is readily soluble in an excess of the 
basic lead acetate, no little care must be exercised when testing 
for mannose in this way. It is easy, however, thus to detect 
mannose when any considerable quantity of it is present, as in the 
product of the hydrolysis by dilute sulphuric acid of date stones, 
the ivory nut or pine wood. After removing all but the Jast 
traces of the acid by neutralizing with lime- or barium-carbonate, 
evaporating to dryness, taking up with warm, strong alcohol and 
applying the lead test after the alcohol has been driven off, a 
satisfactory reaction may be obtained. 
Many attempts were made to apply the test with basic lead 
acetate to the product of the hydrolysis with 5 per cent. hydro- 
chloric acid of substances containing mannan, after having removed 
chlorine by means of moist silver acetate or by a solution of this 
acetate in hot water. In solutions that contain a sufficient quan- 
tity of mannose, the characteristic lead compound may readily be 
precipitated by proceeding in this way, but in dilute solutions the 
reaction is apt to be unsatisfactory ; and since the solution always 
holds dissolved a little silver acetate, — because this substance is 
slightly soluble and because an excess of it is used, — discolora- 
tions will occur through the decomposition of the silver salt when 
any attempt is made to concentrate the liquid by evaporation 
upon a water-bath. It is not difficult, however, to remove the 
silver from the mannose solution by means of clean chips of 
metallic lead, and the fact that on adding a fresh bit of lead it 
remains untarnished shows that the whole of the silver has been 
thrown down. Here again the mannose solution will react freely 
with basic lead acetate, provided it is strong enough, though the 
reaction is feeble if the solution is dilute. It is true that in this 
case the solution can be concentrated upon the water-bath without 
trouble before applying the lead acetate; though, after all has 
been said and done, the liability of vegetable matters to contain 
ash ingredients, notably sulphates, tends in many cases to cast 
a doubt on the lead-mannose reaction unless there is enough man- 
nose present to visibly override the reactions that may be caused 
by traces of impurities. It has been noticed, moreover, in some 
