the
water into a jar of that kind of air with which I would fill the
receiver, I only turn the cock, and I gain my purpose. In this method,
however, unless the pump be very good, and several contrivances, too
minute to be particularly described, be made use of a good deal of
common air will get into the receiver.

When I want to measure the goodness of any kind of air, I put two
measures of it into a jar standing in water; and when I have marked upon
the glass the exact place of the boundary of air and water, I put to it
one measure of nitrous air; and after waiting a proper time, note the
quantity of its diminution. If I be comparing two kinds of air that are
nearly alike, after mixing them in a large jar, I transfer the mixture
into a long glass tube, by which I can lengthen my scale to what degree
I please.

If the quantity of the air, the goodness of which I want to ascertain,
be exceedingly small, so as to be contained in a part of a glass tube,
out of which water will not run spontaneously, as _a_ fig. 15; I first
measure with a pair of compasses the length of the column of air in the
tube, the remaining part being filled with water, and lay it down upon a
scale; and then, thrusting a wire of a proper thickness, _b_, into the
tube, I contrive, by means of a thin plate of iron, bent to a sharp
angle _c_, to draw it out again, when the whole of this little
apparatus has been introduced through the water into a jar of nitrous
air; and the wire being drawn out, the air from the jar must supply its
place. I then measure the length of this column of nitrous air which I
have got into the tube, and lay it also down upon the scale, so as to
know the exact length of both the columns. After this, holding the tube
under water, with a small wire I force the two separate columns of air
into contact, and when they have been a sufficient time together, I
measure the length of the whole, and compare it with the length of both
the columns taken before. A little experience will teach the operator
how far to thrust the wire into the tube, in order to admit as much air
as he wants and no more.

In order to take the electric spark in a quantity of any kind of air,
which must be very small, to produce a sensible effect upon it, in a
short time, by means of a common machine, I put a piece of wire into the
end of a small tube, and fasten it with hot cement, as in fig. 16; and
having got the air I want into the tube by means of the appa