ed it
condenses them, in which condition they are harmless, or they are
retained in the charcoal."

"But how about the impure liquids?"

"Its affinity for sour and stinking liquids is so great that two
tablespoonfuls of charcoal will purify a pint of the foulest sewage; it
will also, in that quantity, absorb 100 cubic inches of gaseous
ammonia."

"Have we anything else that can be used?"

"The sulphur that is in the samples of copper ore, if burned, will make
a sulphurous acid gas, and while it must be carefully used, on account
of its noxious and offensive odor, is a most powerful germicide. Or if
we take some of the green acid of the copper, and make a liquid of it,
and then pour this over common salt we are making what is known as
muriatic acid. The vapor of this acid will destroy all germs. The
objection to this, however, is, that it has an odor which is worse than
the impure or unhealthful gases. In the last samples of ore we brought
home, you may have noticed a very black lot of stuff. That was
manganese. If we take the muriatic acid, which I have just referred to,
and pour it over the manganese, we can make the most powerful agent of
all, namely, chlorine."

"Chlorine is used for bleaching, isn't it?"

"That is its great use in the arts; but as a purifier it has no equal.
It will decompose every gaseous compound and evil-smelling gas which
escapes from decayed animal or vegetable matter."

Harry did not let the Professor forget the matter of the primary
electric battery which he had been making preparations for, and after
they had gone over the sanitary features of their kitchen, he was
anxious to make a start. George was equally insistent, because the
question of a better light was ever uppermost in his mind.

"Then here goes for the primary battery. We haven't any glass, nor have
we found any rubber lying around, so we can make cells out of them, so
what shall we use?" was the Professor's first inquiry.

"Why not use some of these iron pots we made?"

"I am afraid iron would not last very long, with sulphuric acid in them.
We should use some non-conductor of electricity."

"What do you mean by non-conductor?"

"That quality in a material which will not allow electricity to pass."

"Then why can't we use clay? Will that conduct electricity?"

"That is just the thing. Isn't it wonderful what a friend common clay
has been to us since we have been on this island!"

"I think we ought to erect a mo