"You know what happens to starch if we put it into cold water?""Starch is insoluble in cold water," said Norah. "Yes," said Fred, "but if dry starch is baked in anoven it becomes changed. It turns brown, and will easily dissolve in cold water. In this form it is called British gum, the very substance on the back of the postage stamp and the edge of the envelope.
"It is pasted on the stamps and other things with a brush, and dries very quickly. When we want to use it, we wet it, and it will stick fast."SUMMARY
Adhesive substances make other things stick together. We might call them sticky substances. Gum, paste, and sealing wax are adhesive substances for sticking paper. The adhesive substance on an envelope and on the back of a postage stamp is British gum. It is made of starch.
Lesson 16
Cements
"Suppose we have another chat about those adhesive substances," said Fred. "What do we use to stick paper, Norah?""We stick paper with gum, paste, sealing wax, and British gum," said his sister.
"Quite right," said Fred. "Teacher says we may call each of these things a cement. Cement is only another name for an adhesive substance-a substance to make things stick together.