246 ECONOMIC BOTANY. 



(Aurantim), in the order BiUaceoB. It is softer than Gum- 

 Arabic, and entirely soluble in water. The pieces are 

 large and glistening, and have an irregular surface. They 

 are mostly transparent, sometimes dark and yellow, and 

 full of minute clefts. The pieces are sometimes iridescent. 

 It contains more than twelve per cent, of water, and five 

 of ash. It is said to be the best gum for the preparation 

 of water-colors. It finds the same uses as Gum- Arabic. 



210. Gum-Tragacanth is a tasteless and odorless exu- 

 dation from several shrubby plants belonging to the genua 

 Astragalus, of the Pulse or Leguminous family (LeffVr 

 minoscB). In Greece and Crete, A. parnassi and A. (yreticus 

 occur ; and in Asia Minor and Persia, A. verus prevails. 

 The gum exudes spontaneously from the plants, and forms 

 large, dark masses; but usually it is obtained by making 

 incisions. When there are slits, the pieces of gum are in 

 thin plates, striated, and more or less contorted, as is 

 exhibited by that generally found in commerce. Or if 

 punctures in the stem are made, the gum is in the shape of 

 threads. It is collected in July and August. At first it ia 

 soft or half-liquid, but in three or four days it hardens. 

 The translucent yellowish pieces are the best, the lighter 

 pieces are more or less impregnated with air. The color 

 varies from white to brownish-black, some species of plants 

 yielding light, and other species dark gum. It is softer 

 than Gum-Arabic, readily cut with a knife, and can be 

 pulverized after drying at a temperature of 212° F. It 

 swells up in water, but is only partially soluble. When 

 examined under the microscope it exhibits a cellular 

 structure, contains starch grains, etc. This would be 

 expected from the nature of its origin, which is by the 

 metamorphosis of the cellulose of the pith and Medullary 



