英文科学读本 第二册·Lesson 16 Cements

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.

I saw father use some cement of another sort last week. He used it to mend a chair. He put the broken edges of the wood together, with some of this cement between. The chair is quite strong now. The cement has made the broken parts stick fast.

Why, you mean glue, said Norah. "I saw father mending the chair. Of course glue must be an adhesive substance. It is a cement for wood."

Those men who work with bricks and stone, said Fred, "use another sort of cement. Glue would not do."

I suppose you must mean mortar, said his sister.

Quite right, said Fred. "Mortar is an adhesive substance. It is the very sort of adhesive substance to make bricks and stone hold fast together."

What do the men use to make their mortar, Fred? said Norah.

Mortar is made of lime and sharp sand mixed into a paste with water, said Fred.

When it dries it becomes very hard—as hard as the stone itself.

The stone mason, added Willie, "always uses a special sort of mortar. It is made from chalk and lime mixed with clay. He calls it cement. It is mixed up with sharp sand and then made into a paste with water, just as the bricklayer makes his mortar. It is used where great strength is wanted. It becomes as hard as the stone itself when it dries."

SUMMARY

Cements are adhesive substances that stick things together. Glue is a cement for wood; mortar is a cement for bricks and stone.