"I haven't got a dictionary here, uncle," 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 then he glanced at his watch.
"Is that scum the elutriation?" said Tom, with a faint grin.
"No, boy, the impurity; throw it down the sink. Now, Tom, we want to
get our finest polishing emery out of that mixture, and it will take an
hour to form--sixty-minute emery, the opticians call it; so while it is
preparing, we'll go and have another turn at the speculum."
They descended, leaving the pan standing on the heavy table, and after
spreading wet sand upon the lower disc of glass, the loose one was once
more set in motion, and uncle and nephew, with quarter-hour rests for
examination and wetting the surfaces, patiently ground away for an hour,
by which time, upon the speculum being sponged, it was found that the
greater part of the upper glass was deeply scratched.
"This is going to be an awfully long job," thought Tom.
"Yes, it is," said his uncle, who aptly read his thoughts, "a very long
job, Tom; but good things have to be worked for, boy."
"Oh, I'm not going to be tired, uncle. It's like working for a grand
prize."
"It is. Now then, let's see to the emery. Our finest must be ready by
now. Now I want all the water, from which the emery has settled down to
the bottom, drawn off into that great white basin. How is it to be
done?"
"Pour it off," said Tom.
"No; couldn't be done without disturbing the bottom. Let's try
syphoning."
Uncle Richard placed the basin upon a stool below the level of the
table, took up a glass tube bent somewhat in the shape of a long-shanked
hook, placed the short end gently beneath the surface of the nearly
clear water, his lips to the long end, drew out the air, and the water
followed directly from the atmospheric pressure, and ran swiftly into
the basin.
As it ran, and Tom watched, Uncle Richard carefully held the short arm
of the syphon, guiding it till the sediment at the bottom of the pan was
nearly reached, when he quickly withdrew it, and the basin was then
placed beside the pan.
"There, Tom," said Uncle Richard, "that's our sixty-minute emery."
"But I thought you said you wanted it very fine. You've only washed
it."
"We're playing at cross purposes, Tom," said Uncle Richard. "You are
talking about the contents of the pan, I about those of the basin."
"What! the clear water--at least nearly clear?"
"Ah, there you have hit it, boy--nearly clear. That water contains our
finest polishing powder, and it will have to stand till to-morrow to
settle."
"Oh!" said Tom, who felt very much in the dark, and he followed his
uncle to the neat sink that had been fitted in the laboratory, and
helped him wash a series of wide-mouthed stoppered bottles, which were
afterwards carefully dried and labelled in a most methodical way.
"Saves time, Tom, to be careful," said Uncle Richard, who now took up a
pen and wrote upon the label of the smallest bottle "Emery, 60 min."
"There, that's for the contents of the big basin."
"Want a genii to get a pailful into that little bottle, uncle," said
Tom, laughing.
"We'll get all we want into it to-morrow, Tom," was the reply. "Now
then, how do you feel--ready for one hour's more grinding at the
speculum, or shall we leave it till to-morrow?"
"I want to finish it, and see the moon," said Tom sturdily, as he rolled
up his sleeves a little more tightly. "Let's get on, uncle, and finish
it."
"Or get an hour nearer," said Uncle Richard; and they went down and
ground till Mrs Fidler summoned them to their meal.