ten-millionths.

Carbonic acid readily betrays its presence through solutions of the
alkaline earths such as baryta and chalk, in which its passage produces an
insoluble carbonate, and consequently makes the liquid turbid. If, then,
one has prepared a solution of baryta or lime, of which a certain volume is
made turbid by the passage of a likewise known volume of CO_{2}, it will be
easy to ascertain how much CO_{2} a certain air contains, from the volume
of the latter that it will be necessary to pass through the basic solution
in order to obtain the amount of turbidity that has been taken as a
standard. The problem consists in determining the minimum of air required
to make the known solution turbid. Hence the name "minimetric estimation,"
that has been given to this process. Prof. Lescoeur has had the goodness to
construct for me a Smith's minimetric apparatus (Fig. 2) with the ingenious
improvements that have been made in it by Mr. Fischli, assistant to Prof.
Weil, of Zurich. I have employed it frequently, and I use it every year in
my lectures. I find it very practical, provided one has got accustomed to
using it. It is, at all events, of much simpler manipulation than that of
Bertin-Sans, although the accuracy of the latter may be greater (Figs. 3,
4, 5, and 6). But it certainly has more than one defect, and some of the
faults that have been found with it are quite serious. The worst of these
consists in the difficulty of catching the exact moment at which the
turbidity of the basic liquid is at the proper point for arresting the
operation. In addition to this capital defect, it is regrettable that it is
necessary to shake the flask that contains the solution after every
insufflation of air, and also that the play of the valves soon becomes
imperfect. Finally, Mr. Wolpert rightly sees one serious drawback to the
use of baryta in an apparatus that has to be employed in schools, among
children, and that is that this substance is poisonous. This gentleman
therefore replaces the solution of baryta by water saturated with lime,
which costs almost nothing, and the preparation of which is exceedingly
simple. Moreover, it is a harmless agent.

The apparatus consists of two parts. The first of these is a glass tube
closed at one end, and 12 cm. in length by 12 mm. in diameter. Its bottom
is of porcelain, and bears on its inner surface the date 1882 in black
characters. Above, and at the level that corresponds to a volume o