f three
cubic centimeters, there is a black line which serves as an invariable
datum point. A rubber bulb of twenty-eight cubic centimeters capacity is
fixed to a tube which reaches its bottom, and is flanged at the other
extremity (Fig. 7).

The operation is as follows:

The saturated, but limpid, solution of lime is poured into the first tube
up to the black mark, the tube of the air bulb is introduced into the lime
water in such a way that its orifice shall be in perfect contact with the
bottom of the other tube, and then, while the bulb is held between the fore
and middle fingers of the upturned hand, one presses slowly with the thumb
upon its bottom so as to expel all the air that it contains. This air
enters the lime-water bubble by bubble. After this the tube is removed from
the water, and the bulb is allowed to fill with air, and the same maneuver
is again gone through with. This is repeated until the figures 1882, looked
at from above, cease to be clearly visible, and disappear entirely after
the contents of the tube have been vigorously shaken.

The measures are such that the turbidity supervenes at once if the air in
the bulb contains twenty thousandths of CO_{2}. If it becomes necessary to
inject the contents of the bulb into the water twice, it is clear that the
proportion is only ten thousandths; and if it requires ten injections the
air contains ten times less CO_{2} than that having twenty thousandths, or
only two per cent. A table that accompanies the apparatus has been
constructed upon this basis, and does away with the necessity of making
calculations.

An air that contained ten thousandths of CO_{2}, or even five, would be
almost as deleterious, in my opinion, as one of two per cent. It is of no
account, then, to know the proportions intermediate to these round numbers.
Yet it is possible, if the case requires it, to obtain an indication
between two consecutive figures of the scale by means of another bulb whose
capacity is only half that of the preceding. Thus, two injections of the
large bulb, followed by one of the small, or two and a half injections,
correspond to a richness of 8 thousandths of CO_{2}; and 51/2 to 3.6
thousandths. This half-bulb serves likewise for another purpose. From the
moment that the large bulb makes the lime-water turbid with an air
containing two per cent. of CO_{2}, it is clear that the small one can
cause the same turbidity only with air twice richer in CO_{2}, _i.e.