682 [June 18, 



The lengths of two chambers were varied, the radiating column 

 being lengthened, and the absorbing one shortened at the same time. 

 The experiments were carried out with a considerable number of 

 gases and vapours. 



The experiments with the vapours were thus executed. First, 

 the chamber nearest the pile was occupied by vapour of a certain 

 pressure j the other chamber was then occupied by the same vapour 

 at the same pressure. The entrance of the vapour was so slow, and 

 its quantity was so small, that the dynamic radiation due to the 

 destruction of its own vis viva was almost insensible. The needle 

 being at zero, dry air was allowed to enter the chamber most distant 

 from the pile ; the air became heated, communicated its heat to the 

 vapour, and the latter radiated it against the pile. It is quite evi- 

 dent that not only does this case resemble, but it is of the same 

 mechanical nature as that in which a vibrating tuning-fork is brought 

 into contact with a surface of some extent. The fork, which a 

 moment before was inaudible, becomes at once a copious source of 

 sound ; it communicates its motion to a body of sufficient dimen- 

 sions to transmit it in large quantities to the air. What the sound- 

 ing-board is to the tuning-fork, the vibrating compound molecule is 

 to the elementary atom. The tuning-fork swinging alone is in the 

 condition of the elementary atom radiating alone, the sound of the 

 one and the heat of the other being insensible ; but in association 

 with the particles of acetic or sulphuric ether, the elementary atom 

 is in the condition of the tuning-fork applied to its sound-board, 

 communicating through the molecule motion to the luminiferous 

 ether, as the fork through the board communicates it to the air. 



These experiments show the great opacity of a gas to radiations 

 from the same gas. They also show, in a very interesting manner, 

 the influence of attenuation in the case of the vapours. The indi- 

 vidual molecules of a vapour may be powerful absorbers and radi- 

 ators, but in thin strata they may constitute an open sieve, through 

 which a large quantity of radiant heat may pass. In such thin 

 strata, therefore, the vapours, as used in the experiments, were 

 generally found less energetic than the gases, while in thick strata 

 the same vapours showed an energy greatly superior to the same 

 gases. 



A. few striking results are recorded by the author in illustration of 



