Many years ago while making in the laboratory of the late Prof. Carnelley in Dundee, by Pettenkofer's method, a series of Carbonic Acid determinations, in which turmeric paper was used as the indicator, I observed that the last few brown rings made on the paper as the solution approached neutralization showed a tendency to become decolorized in a relatively short space of time, the final ones being seen to vanish even as one looked at them. This at the time—and subsequent observation has confirmed the truth of the conclusion—was ascribed to the CO2 of the air rapidly neutralizing the solution of baryta, which getting more and more dilute as the solution of oxalic acid was gradually added, finally reached a degree of dilution that was all but instantly neutralized by the amount of CO2 present in an ordinary atmosphere. Even then I came to the conclusion that the phenomenon depended on a principle which might probably be made the basis of a rapid method of estimating the amount of CO2 in air generally, but it is only recently that I have had time to adjust the details of the method which I now propose. It will readily be understood that a long series of experiments other than those of which the results are now recorded were carried out before finality was reached on many points of the method. These need not be detailed here, though some of them will be alluded to as occasion arises. It may be stated that the method has been found to give results which approximate to the truth—at least as tested by Pettenkofer's method—much more closely than was at first anticipated.