IlEI'OET OF CHEMICAL LAHORATORA' 
447 
that in a considerable number of cases a gum which soon after being collected yields a 
viscous solution, on being kept for some months becomes much less viscous. These changes 
have also been noted by Fromm, who mentions that this decrease in viscosity is accom¬ 
panied by an increase in the proportion of gelatinous insoluble matter. J)r. Beam’s 
experience of Hashab gums, however, is the reverse in this latter respect — gums which 
at first yielded a ropy solution or a mixture of true solution and gelatinous matter 
invariably gave a true solution on being kept for several months. The gums marked A, B 
and C in the table are instances. In the Second Report of the Wellcome Research 
Laboratories, page 236, Dr. Beam calls attention to the fact that a solution of gum made at 
ordinary temperature has a much greater viscosity than a solution made with hot 
water. I am inclined to think that this change is due to the same cause as the gradual 
decrease in viscosity in a gum which has been kept for a considerable time. This change, 
and the wide range in osmotic pressures, it seems reasonable to attribute to some molecular 
change in the gum, which may he of the nature of polymerisation. I have been unable in 
the time at my disposal to examine these gums in the method described by O’Sullivan* 
in his exhaustive work on the products of decomposition of arabic acid, my object being 
rather to see what differences were obtained by the methods described with gums of 
varying strength. It may be that the nucleus acid to which the sugar residues are 
combined varies with the physical properties of the gum, or there may be varying 
proportions of the sugar residues split ofl' from the different gums when hydrolytic action 
of varying intensity is employed. I think that my figures suggest that on complete 
hydrolysis with 5 per cent, sulphuric acid, however, the products obtained are the same, 
independent of the hardness or viscosity of the gum. 
In his paper on Geddah gum, O’Sullivan found that the number of sugar residues 
attached to the nucleus acid varied, and in some cases the gums contained mixtures of the 
same gum acids, but in different proportions. I am unaware, however, if these gums were 
obtained from the same species of tree, as gum-“ arabic ” from different species of acacia is 
widely different in chemical composition, as I shall show later. 
Even if these Geddah gums were all from the same source, however, it is quite 
reasonable to suppose that in Hashab gum the changes in physical properties when the 
gum is merely kept at ordinary temperatures is due to some molecular change, as it is 
improbable that changes would occur in the actual proportions of the sugar residues or the 
gum acids. 
If freshly-collected gum from various sources were available it would be very 
interesting to examine it chemically and physically at intervals of a few weeks, so as to 
discover if any such differences occur such as O’Sullivan found in the case of Geddah gum, 
and also if the osmotic pressure and viscosity varied together. Unfortunately none of the 
newly-collected gum of last season was available. 
Talk Gum 
I have examined a few samples of Talh gum from different sources — one the so-called 
“ White Talh,” the others Red Talh probably, though information as to the source was not 
always detailed. It is interesting to note that each sample has nearly the same composition 
and optical activity, so that probably molecular changes account for variations in hardness 
and viscosity, as suggested in the case of Hashab gums. 
Molecular 
change in 
gums 
Talh gum 
Jtmvnal of Ihr. Chemical Sociefif (Trnusactiuus). XTjV., p. *41. 
