§§ 90, 91. SOLUBLE MODIFICATION OF ARABIC ACID. 77 



generally be present that are soluble in water, not precipitated by 

 alcohol or neutral acetate of lead and have up to the present 

 time eluded investigation. It might appear hazardous to make 

 conjectures as to the nature of these substances, but I cannot help 

 remarking that in some cases a substance seemed to me to be 

 present which, after evaporation of its alcoholic or aqueous solu- 

 tion, did not again dissolve completely in either of those liquids. 

 It appeared to agree in some of its properties -with that form of 

 vegetable mucilage that is obtained by dialyzing acidified solutions 

 of gum, etc., which sometimes remains in solution on the addition 

 of alcohol. When I have met with a substance agreeing with 

 mucilage in this peculiarity, I have spoken, it is true, of a ' soluble 

 modification of arabic acid,' but I have not omitted to place a 

 query after it.^ The further investigation of this substance is a 

 desideratum for plant-analysis. 



But in thus assuming the presence of such an ' arabic acid,' 

 account must be taken of the results of the nitrogen determina- 

 tions to be described in § 96. By deducting the nitrogen in the 

 residue of the material after extraction with water from that in 

 the original substance, the amount in the sul)stances soluble in 

 water is ascertained. If, now, the amount of nitrogen present as 

 albuminoids, nitric acid, ammonia and alkaloid is calculated from 

 the separate determinations and found to be much smaller than 

 the estimation by difference, it should be remembered that under 

 certain conditions water may dissolve albuminoids which alcohol 

 fails to precipitate. 



§ 91. Mannite. — Another substance, however, which is of not 

 unfrequent occurrence in the vegetable kingdom, would similarly 

 elude detection by the foregoing experiments with the alcoholic and 

 aqueous extracts, as it is almost insoluble in cold absolute alcohol 

 but is not precipitated from its aqueous solution by the addition 

 of either spirit or lead salts. The substance referred to is mannite. 

 If present it would be included in the deficit mentioned in § 90, 

 but would be easy of detection, as it crystallizes with great facility 

 in long prisms and needles and is somewhat sparingly soluble in 

 cold spirit. It may be approximately estimated by precipitating 

 the aqueous solution with alcohol and basic acetate of lead, 



^ Compare my 'Chem. Beiträge z. Pomologie,' Dorpat, 1878 ; Verlag d. Dor- 

 pater Naturforscher Gesellsch. ; and Pfeil, ' Chem. Beiträge z. Pomologie,' 

 Diss. Dorpat, 18S0. 



