“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.