102 



Below a certain tension, which varies for the different gases, the 

 amount of heat absorbed is exactly proportional to the density of 

 the gas. 



Above this tension the rays on which the principal absorptive 

 energy is exerted become gradually exhausted, so that every aug- 

 mentation of density produces a diminished effect. 



In the case of olefiant gas, for example, a unit-measure -^th of a 

 cubic inch in capacity being made use of; for a series of fifteen such 

 measures the absorption was exactly proportional to the quantity of 

 gas ; subsequently the ratio of the successive absorptions approached 

 gradually to a ratio of equality. The absorption produced by a 

 single measure of olefiant gas of the above volume moved the index 

 of the galvanometer through an angle of 2-2 degrees, the tension of 

 the gas being only yy-oirtj^ f an atmosphere. 



In the case of vapours, the most energetic is that of sulphuric 

 ether ; the least energetic is that of bisulphide of carbon. Com- 

 paring small volumes at equal tensions, the absorptive energy of 

 sulphuric ether vapour is ten times that of olefiant gas, and ten 

 thousand times that of oxygen, hydrogen, nitrogen, or atmospheric air. 



On a fair November day the aqueous vapour in the atmosphere 

 produced fifteen times the absorption of the true air of the atmo- 

 sphere. It is on rays emanating from a source of comparatively low 

 temperature that this great absorptive energy is exerted ; hence the 

 aqueous vapour of the atmosphere must act powerfully in intercept- 

 ing terrestrial radiation ; its changes in quantity would produce cor- 

 responding changes of climate. Subsequent researches must decide 

 whether this vera causa is competent to account for the climatal 

 changes which geologic researches reveal. 



Oxygen obtained from the electrolysis of water exerted four times 

 the absorptive energy of the same substance when caused to pass 

 through iodide of potassium ; the greater action being due to the 

 presence of ozone. 



The radiative power of gases was examined by causing them to 

 pass over a heated sphere of metal, and ascend in a column in front 

 of the thermo-electric pile ; various precautions were taken, which 

 are fully described in the memoir. It was found that the order of 

 radiation was exactly that of absorption ; that any atom or molecule 

 which is capable of accepting motion from agitated ether, is capable, 



