le," said Tom, with a smile, as
they stood at the massive table under the window in the laboratory. "I
don't know what elutriation means."

"I dare say not. I didn't till I was nearly fifty, Tom, but you soon
shall know. Fetch that tin off the shelf."

Tom obeyed, and found a label on the top, on which was printed "Best
Ground Emery."

"Well, you know what that is?"

"Emery? Powdered glass," said Tom promptly.

"Wrong. Diamond cuts diamond, Tom, but we want something stronger than
powdered glass to polish itself. Emery is a mineral similar in nature
to sapphire and ruby, but they are bright crystals, and emery is found
in dull blocks."

"Then it's very valuable?" said Tom.

"Oh, no. It is fairly plentiful in Nature, and much used. Now then, we
want coarse emery to grind our speculum after we have done with the
sand, and then different degrees to follow, till we get some exquisitely
fine for polishing. How are we to divide the contents of that tin so as
to graduate our grinding and polishing powder?"

"Sift it, of course, uncle."

"And where would you get sieves sufficiently fine at last?"

"Muslin?"

"Oh, no. Here is where elutriation comes in, Tom; and here you see the
use of some of the things I brought back from London the other day. To
work. Bring forward that great pan."

This was done.

"Now empty in the contents of this packet."

Tom took up a little white paper of something soft, opened it, and
poured the contents into the pan.

"Powdered gum arabic?" he said.

"Yes. Now empty the tin of emery upon it."

Tom opened the tin, and found within a dark chocolate-looking powder,
which felt very gritty between his finger and thumb. This he emptied
upon the gum arabic, and, in obedience to instructions, thoroughly mixed
both together.

"To make the fine emery remain longer in suspension," said his uncle,
"keep on stirring, Tom."

"All right, uncle. What, are you going to pour water in? It's like
making a Christmas pudding."

For Uncle Richard took up a can of water, and began to pour a little in
as Tom stirred, changing the powder first into a paste, then into a
thick mud, then into a thin brown batter, and at last, when a couple of
gallons or so had been poured in and the whole well mixed, the great pan
was full of a dirty liquid, upon the top of which a scum gathered as the
movement ceased. This scum Uncle Richard proceeded to skim off till the
surface was quite clear, and