The following pages contain an account of experiments made by myself and Mr T. Lindsay, at the request of Professor Tait, to determine the absorption of damp air containing a known quantity of water vapour, by finding in what proportions dry air and olefiant gas must be mixed in order that the absorption of the mixture may be the same as that of the damp air. The experiments were made according to the method described by Professor Tait in a letter read by Sir William Thomson at the Southampton meeting of the British Association (see Nature, vol. xxvi. p. 639, 1882). They consist of two series—the first, a series of rough experiments made, first, to determine whether or not the method would work, and, secondly, to find the dimensions of the apparatus which would give the best result; the second, a series of more careful experiments made when the method was found to give satisfactory results.