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,"