ON THE rURSKNr rOSITTOX OF THE CHEMISTRY OF THE GUMS. 231 



of the sugar residues are split off and acids of lower and lower molecular 

 weight and of higher and higher neutralising power are obtained, until 

 arable acid is reached. This acid is of considerable stability and requires 

 strong hydrolytic action to break it up, and it then appears to suffer a 

 great degree of disintegration. In the light of the pentose nature of 

 arabinose announced in 1887 by Kiliani, O'Sullivan assigns to the gum 

 substance the formula 2CioH|f,Oj„ 4C,2H.,(,0,o, CasHjoOig, and names it 

 di-arahinan-tetrayalactan-arabic acid. In this ' arabinan ' and ' galactan ' 

 stand for two molecules of arabinose and galactose respectively minus 

 two molecules of water, the termination ' an ' indicating the anhydride of 

 the corresponding sugar. The arable acid nucleus appears in this formula 

 ■with four molecules of water less than in the free acid, indicating the 

 occurrence of four double oxygen attachments between the sugar residues 

 and the nucleus acid, and showing that sixteen molecules of water are 

 required for complete hydrolysis to arabinose, galactose, and free arable 

 acid. In the natural gum, of course, the acid is more or less neutralised 

 by the potash, lime and magnesia of the ash constituents of the plant. 

 The * arable acid ' of O'Sullivan thus denotes the nucleus acid, and not the 

 acid of the natural gum substance ; since the latter acid differs in different 

 specimens of gum, this seems to be an advantageous change, especially as 

 it permits of descriptive names being given to the varying natural gum 

 acids. 



A gum known in commerce as Geddah gum was next examined by 

 O'Sullivan ; it resembles gum arable in being soluble in water, but it is 

 dextrorotatory, whilst gum arable is lievorotatory. He found that it 

 is a mixture of several gum acids, which are constituted of the radicles of 

 galactose and of arabinose or arabinon, attached in considerable numbers 

 to a nucleus acid to which the name geddic acid is given. Geddie acid is 

 an isomer of arable acid, C .3113,0.22. The acids forming the mixture of 

 which the gum itself is composed are of high molecular weight, and differ 

 from one another in the number of arabinon molecules they contain and, 

 consequently, in their rotatory power. 



The third gum investigated was tragacanth. This, like Geddah gum, 

 was found to be a mixture of several gum acids. It can be separated 

 into a group of acids which remain in solution in dilute alcohol, and an 

 insoluble portion, for which the old name bassorin is appropriated. The 

 acids of the soluble group were found to be built up on a nucleus acid 

 very similar, if not identical, with geddic acid, by its union with galac- 

 tose and arabinose residues. The constitution of the insoluble portion, 

 bassorin, has not yet been completely worked out, but it yields a nucleus 

 acid of the formula C,4H2nO,3, to which the name bassoric acid is given, 

 and intermediate acids formed of this acid united to the residues of 

 xylose and of a new pentose sugar named tragacanthose. 



It has been observed that in the case of the gum from sugar-beet the 

 gum obtained in one season frequently differs in rotatory power from that 

 obtained in another season, and that in the case of gum arable different 

 samples also differ in this respect. The explanation of this was discovered 

 in the work on Geddah gum ; the cause lies in the fact that in these 

 varying gums the number of sugar residues attached to the nucleus acid 

 varies, or in some cases it may be that the gums contain mixtures of the 

 same gum acids, but in different proportions. 



It was found that in a series of gum acids containing arabinans and 

 galactans in varying numbers joined to geddie acid that there were 



