GUM EESEARCH 67 



Varieties of hashdh (jum. The gum found exuding naturally is called by the natives Varieties of 

 " wady " gum and is usually held by them to be stronger than that resulting from '"*'"* g'"" 

 tapjoing. This is iiot borne out by the result of examination. Indeed, wady gum is, 

 on the whole, of inferior quality, since it is usually darker in colour than the gum of 

 tapped trees. 



After tapping, the first collection of gum is usually made at the end of several 

 weeks, more or less, depending in part at least irpon the temperature, which has a 

 marked influence upon the rate of exudation. There are usually seven or eight collections 

 in the season, which terminates when the rains begin to fall. 



The gum of the earliest exudation is usually not completely soluble. On treatment 

 with water, a glairy, mucus-like liquid results, from which there separates, after a time, 

 a greater or less amount of true solution. This effect is usually less marked in weak 

 solutions. Fortunately it has been found that storage of the gum for a short period is 

 sufficient to correct this defect, the gum becoming completely soluble in solutions even 

 of the highest concentration. Senegal gum has been found to behave in a similar manner. 



The change which takes place in the gum on storage has not yet been worked out, 

 hut it is doubtless connected with the presence of certain enzymes' which have been 

 found to be present. 



llashdh {A. Senegal) gum is found to be a mixture of two markedly different varieties, 

 one hard, and the other soft. These may be separated by exposing the mixed gum to 

 the sun. The harder variety remains glass-like and transparent, whereas the softer 

 becomes more or less rapidly covered with innumerable small fissures which make it 

 appear almost quite white. This bleaching of the gum is more apparent than real, and 

 if the gum is originally appreciably coloured it will remain so in the interior. A certain 

 amount of true bleaching does, nevertheless, take place, but it is not very marked. The 

 solution of this variety of gum is less viscous than that of the harder variety. 



The usual range of viscosity of 20 per cent, solutions of the two varieties is as follows, 

 the figures expressing degrees of retardation in the torsion viscosimeter.- 

 Hard gum ... ... ... ... 60 to 70 degrees 



Soft gum 28 to 33 ,, 



The two gums exhibit no apparent differences as regards their proportion of ash 

 and acidity. 



The origin of the hard gum, that is to say, the special conditions which determine 

 its formation, has not yet been determined. A great deal of work has been attempted 

 with this end in view, but the investigation had to be abandoned for a time for the 

 reasons stated above. 



If tested shortly after exudation, gum of the six or more successive collections in a 

 given season exhibits a more or less regular decrease in apparent viscosity strength of 

 solutions. This is not due to a higher proportion of the hard, transparent, strong gum 

 in the earlier collection but to the presence of the less soluble, constantly changing 

 variety to which reference has already been made. If the gum is stored for some 

 months before testing, the change to complete solubility will usually bo found to have 

 taken place, and all the collections will exhibit approximately the same viscosity strength. 



The typical hard, strong, gum suffers but little change in viscosity strength, on storing. 



Effect of coppicing. A considerable proportion of hashdh gum is derived from coppiced Effect of 

 trees. As such gum might show marked difference in quality or quantity from that <=°PP"='"R 



' Vide F. Rciuitzer, Zeit f. Physiolog. Clwm. 1909, p. 352 



- Vide Second /ieport, JP'eUcome Tropical Research Lnbornlories, p. 232 



