of Acids by Carbohydrates. 549 



(c) Carbohydrates, whose molecules contain less than six 

 atoms of carbon (arabinose, glj^collic aldehyde) gave brown or red 

 colours without any appearance of purple, and the polyhydric 

 alcohols showed only a yellow colour. 



(d) Carbohydrates like cellulose and starch gave a very faint 

 red, or no colouration at all. 



In a later communication (/. C. S. Trans. 1899, p. 423), the 

 same authors showed that the product of the action of hydrogen 

 bromide on the various carbohydrates was a colourless, crystalline 

 substance, bromomethylfurfuraldehyde. 



The action of hydrogen chloride was found by them to be 

 similar to that of hydrogen bromide, but the reaction was much 

 less violent, and the yield of chloromethylfurfuraldehyde not 

 nearly so good. 



Subsequent experiments shewed that all ketoses, or sub- 

 stances which give rise to these on hydrolysis, yield bromo or 

 chloromethylfurfuraldehyde under this treatment ; the aldoses 

 give mere traces whereas the ketoses yield from 20 — 30 per 

 cent., so that by estimation of the yield a ready method is afforded 

 of distinguishing the two classes. 



Principle of the Method. 



A weighed quantity of the various carbohydrates, finely 

 powdered, was treated with an excess of a standard solution of 

 dry hydrogen chloride gas in dry chloroform or carbon tetrachloride, 

 and the mixture was allowed to remain for some hours in tightly 

 stoppered bottles, at a constant temperature. 



The clear liquid was decanted from the solid, a known volume 

 shaken with much water and titrated against standard baryta, 

 phenolphthalein being used as indicator. 



From this experiment the weight of hydrogen chloride adsorbed 

 by the solid in each case was easily deduced. 



It was found advisable to cover the solid with a known amount 

 of pure solvent (generally 5 c.c), in order to allow the former to 

 absorb the maximum amount of liquid before the addition of the 

 standard acid solution. 



ExperiTnental. 



Experiment I. 



Weight of each carbohydrate used, 025 gm. 

 Time of experiment, 18 hours. 

 Mean temperature, 14° C, 



36—3 



