HEl’OKT OF CHEMICAL LAHOKATOHY 
443 
The gum in the first six instances above was from thirty middle-sized trees in Slieikh 
Ali Nur-ed-l)in's garden, of wliich a more detailed account is given by Dr. Beam. The 
exact source or description of the trees in the case of the other gums is not known, except 
that they are from Kordofan and Senegal. The hard, glassy gum from Kordofan was a 
picked sample with a high viscosity. One of the samples was a so-called “bleached” 
gum after exposure to the sun. This, as Ur. Beam has pointed out, is not a true bleaching, 
the white effect being due to the presence of a large number of cracks and small fissures 
in the gum. This gum is very friable. 
The last specimen of gum mentioned yielded a ropy solution when it was first examined 
in the laboratory, but now a true solution is obtained which develops no ropiness on 
standing over-night. 
It may make the tables clearer if the method of calculating the glucoses is illustrated 
by an example. In the first case the actual weight of phloroglucide obtained on adding 
phloroglucin to the furfurol resulting from tlie distillation of one gramme of gum with 
hydrochloric acid was 0-31S4 grammes. From tables given in Kroher’s paper this is found 
to correspond to 0-3537 grammes of arabinose, which will reduce a quantity of Fehling’s 
solution equivalent to 0-856 grammes of cupric oxide. The total reduction by one gramme 
of gum after hydrolysis was 2-005 grammes cupric oxide, so that there is a reduction of 
1-149 grammes CuO due to glucoses or galactose. The amount of glucoses required for this 
reduction is 0-5222 grammes, or 52-22 per cent, of the gum. 
These results, especially in the series of six samples of gum from the same set of trees, 
are at first sight surprising, as they show that, in spite of the different season of collection 
and differences in the proportions of friable and hard glassy gum, the products of hydrolysis 
may be said to be the same throughout. The differences in reduction of cupric oxide are 
multiplied five-fold in the table, as the solution taken only contained 0-2 grammes of gum. 
No allowance, either, is made for differences in moisture, ash, adherent hark, etc. 
The other six samples show that the proportions of pentoses and hexoses remain 
practically the same in spite of locality or age of the gum. It would have been interesting 
to examine the gum of Acacia verek, from other sources than the Sudan or Senegal, but 
there were no specimens in the laboratory. Some gums collected two years ago gave the 
same figures as specimens collected in the early months of 1908. Several other samples of 
gum were examined in the same manner, and gave results within the limits mentioned in 
the table. 
An important feature in the recognition of gums is their optical activity. Fromm' 
states that the rotation due to a 10 per cent, solution of air-dried “Kordofan” or 
“Senegal” gum varies from about —1“ to + 3*^ 21' in a tube 100 mm. in length, with 
variations in abnormal cases ranging from almost —5“ to over + 9*^. lie mentions that 
the best varieties without exception give a negative reading, but I have found that all 
samples of Hashab gum examined gave a negative reading, and within much narrower 
limits than Fromm gives. Greig Smith, in his work on the bacterial origin of gums, 
mentions that according to various authors the natural gums vary constantly in their 
optical activity, but I have only found this to be so in the case of gums from different 
species of trees. 
I have also estimated the optical activity of a solution of gum after hydrolysis. The 
gum was hydrolysed as described for the estimation of total reducing sugars, and the 
rotation in a 100 mm. tube observed. From this the rotation of a 10 per cent, solution of 
gum after hydrolysis was calculated. 
Calculation 
of the 
glucoses 
Pentoses and 
hexoses 
Optical 
activity 
of gums 
' Zciis.f. Anal OhcmiCy 1901, XL., p. 143. 
