78 



REPORT OF CHEMICAL LABORATORY 



Technique for 

 determination 

 of acidity and 

 viscosity 



" Bleached " 

 appearance 

 due to cracks 

 and fissures 

 of surface 



gum then usually coi:tainecl about 10 per cent, of moisture. The acidity was determined 

 by titrating a weighed amount (about 3 grammes) with decinormal sodium hydrate, 

 using phenol-phthalein as an indicator. The results are expressed as milligrammes of 

 potr^ssium hydrate required to neutralise one gramme of gum. The acidity was always 

 a little liigher in the case of the gum from the small trees, but this is unimportant. 



The viscosity was determined by using Ostwald's viscosimeter as described by 

 Dr. Beam in the Third Eeport of these Laboratories. The solutions used contained 20 

 grammes of dry gum in 100 c.c, allowance being made for the percentage of moisture in the 

 particular sample under examination. The determinations of viscosity were made at 320., 

 and the results ai'e expressed in terms of the amount of pure cane-sugar in 100 c.c. 

 required to give the same viscosity. The gum of the third collection from the 20 small 

 trees was slightly ropy when first tested, but in the course of another month it had 

 become completely soluble. All the other samples tested gave a homogeneous solution. 

 The gum in all cases compared favourably with good commercial samples, giving an 

 almost white powder, and a 20 per cent, solution had usiially a very pale straw colour. 



As a rule, the gum from the small trees had a higher viscosity than that of the same 

 collection from the large trees, but this was not invariably the case. I tested the gum 

 of the earlier collections at intervals during the season, and found a gradual falling off 

 in the viscosity, this being most marked in the gums of the third and fourth collections. 

 On being kept in the sun for a short time a large proportion of all the gum took on the 

 "bleached" appearance of the commercial grade of gum which is exposed to the sun in 

 large quantities at Omdurman. As Dr. Beam has previously remarked, this is not a 

 real bleaching, the appearance being due to the presence of a very large number of minute 

 cracks and fissures on the surface of the tears. This does not occur in the case of the hard, 

 strong gum of high viscosity. Dr. Beam mentioned in his last report that this strong 

 high-grade gum was said to occur more in the collections made very early in the season, 

 about Novem,ber or December. In the year 1908, however, there was no gum brought 

 into Taiara in the former month, and only a small quantity in December. I was 

 given to understand also that this was the usual condition of affairs there. It may 

 be that in other districts of Kordofan the collection of gum begins at an earlier date, 

 and Dr. Beam, I believe, arranged for a series of collections to be made in different 

 parts with a view to determining whether the presence of hard, strong gum depended 

 on local conditions, such as soil, height and water level in the ground, etc. 



I*aradoxical 

 result of 

 attempts to 

 infect the site 

 of tapping 



The Effects of Treatment of Rashdh Trees on the Yield of Gum 



In the Third Report of the Wellcome Tropical Research Laboratories, Khartoum, page 422, 

 Dr. Beam gives the results of experiments carried out by him in which he tried the effect 

 of rubbing a moist rag over the bark and then into a cut which was made in the tree 

 instead of the bark being removed. This was done in view of Greig Smith's work on the 

 production of gum by bacteria, the object being to facilitate the entrance of the bacteria 

 to the wound. The result was a complete surprise, however, the yield of gum being 

 considerably less in tlie case of the trees thus treated than in the control trees. Dr. Beam 

 concluded from this that the rubbing had washed away much of the sap containing the 

 bacteria and therefore rendered infection less complete. This would probably be the effect 

 of such treatment in those cases where only a few bacteria were already present in the 

 tree, and in other cases, as Dr. Beam has also pointed out, the effect would be to wash 

 away the sweet sap, after which there would be less attraction for the ants and flies which 

 carry the infection. At the same time it nmst be remembered that trees vary enormously 



