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)