434 VEGETABLE SUBSTANCES. 



CHAPTER XXVII. 

 GUMS. 



ARABIC SENEGAL TRAGACANTH KUTEERA CAROB 

 STARCH. 



GUM is a thick transparent tasteless fluid, which gra- 

 dually hardens without losing its transparency. It 

 exudes from certain species of trees, and from its ad- 

 hesive quality is extremely useful in the arts. It is 

 moderately hard and somewhat brittle, so that when 

 cold it may readily be reduced to a fine powder. 

 This substance is extremely soluble in water, but 

 insoluble in alcohol, being exactly opposed in this 

 respect to the resins. On the application of heat it 

 swells and softens : it is infusible. 



Gums are largely used in topical or calico printing 

 to give a proper consistency to the cloth previously to 

 the application of the mordants, by which means they 

 can be evenly laid on the surface and are prevented 

 from running and mixing with each other, and thus 

 rendering the pattern indistinct and imperfect. 



Gum Arabic is most extensively employed for this 

 purpose. The common appearance of this gum is too 

 well known to need a particular description ; when 

 of a pale yellow colour it is most esteemed. 



It is obtained from the Acacia nilotica, or Egyp- 

 tian acacia. This tree is not of very large growth, 

 rising only to somewhat beyond twelve feet in height. 

 The bark of the trunk is smooth and of a grey colour, 

 that of the branches has a slight purple tinge. The 

 leaves are double-winged, and placed alternately on 

 the branches. The single leaves grow in pairs on 

 stalks, which are likewise placed in pairs opposite to 

 each other on the thicker stalk that grows out from 

 the branches. Long white spines proceed from 

 each side of the base of the leaves. The flowers are 



