There is a large number of processes and apparatus for estimating the
amount of carbonic acid in the air. Some of them, such as those of
Regnault, Reiset, the Montsouris observers (Fig. 1), and Brand, are
accurate analytical instruments, and consequently quite delicate, and not
easily manipulated by hygienists of middling experience. Others are less
complicated, and also less exact, but still require quite a troublesome
manipulation--such, for example, as the process of Pettenkofer, as modified
by Fodor, that of Hesse, etc.
[Illustration: APPARATUS FOR ESTIMATING THE CARBONIC ACID OF THE AIR.
FIG. 1.--Montsouris Apparatus. FIG. 2.--Smith's Minimetric Apparatus. FIG.
3.--Bertin-Sans Apparatus. FIG. 4.--Bubbling Glass. FIG. 5.--Pipette. FIG.
6.--Arrangement of the U-shaped Tube. FIG. 7.--Wolpert's Apparatus.]
Hygienists have for some years striven to obtain some very simple apparatus
(rather as an indicator than an analytical instrument) that should permit
it to be quickly ascertained whether the degree of impurity of a place was
incompatible with health, and in what proportion it was so. It is from such
efforts that have resulted the processes of Messrs. Smith. Lunge,
Bertin-Sans, and the apparatus of Prof. Wolpert (Fig. 7).
It is of the highest interest to ascertain the proportion of carbonic acid
in the air, and especially in that of inhabited places, since up to the
present this is the best means of finding out how much the air that we are
breathing is polluted, and whether there is sufficient ventilation or not.
Experiment has, in fact, demonstrated that carbonic acid increases in the
air of inhabited rooms in the same way as do those organic matters which
are difficult of direct estimation. Although a few ten-thousandths more of
carbonic acid in our air cannot of themselves endanger us, yet they have on
another hand a baneful significance, and, indeed, the majority of
hygienists will not tolerate more than six ten-millionths of this element
in the air of dwellings, and some of them not more than five
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 of 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 5½ 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._, of
four per cent.
This apparatus, although it makes no pretensions to extreme accuracy, is
capable of giving valuable information. The table that accompanies it is
arranged for a temperature of 17° and a pressure of 740 mm. But different
meteorological conditions do not materially alter the results. Thus, with
10° less it would require thirty-one injections instead of thirty, and
CO_{2} would be 0.64 per 1,000 instead of 0.66; and with 10° more, thirty
injections instead of thirty one.
The apparatus is contained in a box that likewise holds a bottle of
lime-water sufficient for a dozen analyses, the table of proportions of
CO_{2}, and the apparatus for cleaning the tubes. The entire affair is
small enough to be carried in the pocket.--_J. Arnould, in Science et
Nature_.
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[NATURE.]