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.