76 SUBSTANCES SOLUBLE IN WATER. 



heating a solution of 1 part in 12 or 13 of water with 1*5 to 2'0 

 parts per mille of sulphuric acid (specific gravity 1*12), in sealed 

 tubes, to 130 or 135. The estimation of sugar by the fermenta- 

 tion of such solutions is said to yield numbers that are rather too 

 low. That is not the case with determinations by Fehling's or 

 Knapp's method. 



I am, on the whole, more inclined to use hydrochloric acid for 

 inverting ; but if the acid is to be subsequently removed, I must 

 acknowledge that sulphuric is to be preferred, as it is easily pre- 

 cipitated by carbonate of barium. 



88. Bottger's Test. The above tests also suffice for the detec- 

 tion of glucose and saccharose. Bottger's bismuth test may be 

 employed as confirmatory of the presence of the former. It consists 

 in warming the liquid with a solution of carbonate of soda, to- 

 gether with oxynitrate or hydrate of bismuth ; if sugar be present, 

 grey suboxide of bismuth is formed. (See also 200.) 



89. Distinctive Characteristics. The chief marks of distinction 

 between the various members of the glucose or of the saccharose 

 group are to be found in the difference in crystalline form, etc., 

 and in the action on polarized light. In the cases here alluded to, 

 use may sometimes be made of these characters, especially if the 

 solution contains only one carbohydrate and no other substance 

 that might influence the crystallization or optical activity. But 

 these conditions are seldom fulfilled, and in the majority of cases 

 we must, therefore, forego an exact identification of the particular 

 glucose and saccharose present, unless we have a considerable 

 quantity of the substance under examination at our disposal. 

 (Ci 205-207.) - 



If we have command of a large quantity of material, it would 

 be best to endeavour to effect the separation of the carbohydrates 

 by treatment with different solvents, decolourization with animal 

 charcoal and crystallization. The crystallization of glucose is 

 favoured by direct sunlight ; the presence of a small quantity of 

 a mineral (hydrochloric) acid may also prove advantageous. (See 

 also 205-207.) 



90. Soluble Modification of Arabic Acid. Albuminoids not Pre- 

 cipitated by Alcohol. In almost every plant-analysis the sum-total 

 of the separate estimations of the substances soluble in water 

 (mucilage, etc.) will be found lower than the estimation of the 

 total solids in solution. One or more substances must, therefore, 



