The object of the paper was to examine into the present state of our knowledge of this subject, for the purpose of ascertaining its defects, and indicating the manner in which they were to be remedied.

Although it appears to any one who examines only a single authority on the constitution of vapour, that its laws are sufficiently definite, yet on comparing together all the authorities we possess on this subject, or even the best of them, we find such discrepancies both in the general expressions of the law, and in the experiments themselves upon which they rest, as are highly discreditable to mechanical science, and far exceed the limits of error usually admitted in similar departments of experimental science. Between 32° and 212°, the difference between Dr Dalton's experiments and Dr Ure's amounts to more than three-tenths of an inch of mercury. Between the experiments of the Institute of France and of the Franklin Institute,—the latest and most extensive series of experiments,—there is a difference at the eleventh atmosphere of more than 6° in temperature, and about 20 inches of mercury in the pressure of the vapour.