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instead of paste and glue; and, as it can be made of a thick 

 consistency, it is used in calico-printing to thicken colors 

 and mordants. Some of the gums, as gum-tragacanth, do 

 not dissolve in water, but only swell up, forming a jelly. 

 The juices of many fruits and roots as currants, cherries, 

 apples, carrots, etc. contain a peculiar kind of gum, which 

 gives to the juices the property of hardening into a gelati- 

 nous mass on cooling, especially when they have been boiled 

 with sugar. This is called pectin. 



454. Dextrin. Starch, by certain processes, may be con- 

 verted into a gum called dextrin, C 6 H 10 O 5 . The change 

 can be produced simply by the action of heat. If starch 

 be roasted, being kept in motion all the while to prevent 

 its burning, it becomes at length of a brownish-yellow color, 

 and then has the property of being soluble in either cold or 

 hot water. This dextrin, or starch -gum, as it is common- 

 ly called, is much used in calico-printing for thickening 

 colors and mordants, and is prepared for this purpose in 

 large quantities by roasting it in the same way that coffee 

 is roasted. This gum is also used in making " fig-paste " 

 and other kinds of confectionery. 



Dextrin can be made with a less degree of heat by the 

 agency of either sulphuric or nitric acid. Let a paste of 

 potato starch be made, and while it is yet hot drop a few 

 drops of sulphuric acid upon it in a saucer, and stir it in 

 well. You will find that the swollen starch soon becomes 

 liquid. Now place the saucer upon a jar in which water 

 is simmering, and let it remain over the steam until the 

 liquid is nearly transparent. You have now a solution of 

 dextrin, with the sulphuric acid in it unaltered. To rid 

 it of the acid you add prepared chalk until effervescence 

 ceases. There is gypsum now in the solution in place of 

 the sulphuric acid, for the acid has united with the lime of 

 the chalk, setting free its carbonic acid, which, decomposing 



