Heat is evolved in air when its motion is arrested; on entering an exhausted tube, the more rapid the motion the greater the heat. Both chambers of the cylinder were at first filled with the vapour to be examined, the usual pressure being the 1/60 part of an atmosphere. But the vapour entered so slowly, and the quantity was so small that the radiation due to the warming of the vapour by its own collision was insensible. The needle of the goniometer being at zero, dry air was allowed to enter the chamber most distant from the pile; this air became heated dynamically by the collision of its particles against the sides of the tube, communicated its heat to the vapour, and the vapour immediately discharged the heat thus communicated to it against the pile. This case not only resembles, but is actually of the same mechanical character as, that in which a vibrating tuning fork is brought into contact with a surface of some extent. The fork, which before was inaudible, becomes at once a copious source of sound. What the sounding board is to the fork, the compound molecule is to the elementary atom. The tuning fork vibrating alone is in the condition of the atom radiating alone; the sound of the one and the heat of the other being insensible. But in association with sulphuric or acetic ether vapour the elementary atom is in the condition of the tuning fork applied to its sounding board, communicating motion to the luminiferous ether through the molecules, as the fork through the board communicates its motion to the air. Mr. Tyndall’s experiments show the great opacity of a gas to radiations from the same gas, and may likewise show the remarkable influence of attenuation in the case of vapour. The individual molecules of a vapour may be powerful absorbers and radiators, but in their strata they constitute an open sieve through which a great quantity of radiant heat may pass. In such thin strata, therefore, the vapours as used in the experiments were generally found far less energetic than the gases, while in thick strata the same vapours showed an energy greatly superior to the same gases, but the gases were always employed at a pressure of one atmosphere. Lastly Mr. Tyndall examined the diathermancy of the liquids from which his vapours were derived, and the result leaves not a doubt that both absorption and radiation are phenomena irrespective of aggregation. If any vapour is a strong absorber and radiator, the liquid from whence it comes is also a strong absorber and radiator.