Radiation of Heat by Gaseous Matter. 345 



of the atom, or some other attribute than its period of oscilla- 

 tion, must enter into the question of absorption. It is physi- 

 cally incapacitated from communicating motion, and hence in an 

 equal degree incapacitated from accepting motion. The neutra- 

 lity of elementary gases in the experiments on absorption already 

 recorded does not arise from my accidentally choosing a source 

 of heat whose periods do not synchronize with those of the gas ; 

 for however they might synchronize, the gas would still be a bad 

 absorber. Even when the motion which iheir own absorbent 

 power does not enable them to take up is mechanically imparted, 

 or is communicated to them by contact, they are still incom- 

 petent to expend it upon the ether, which accepts all vibrations 

 alike*. 



. § 9. Scents and effluvia generally have long excited the atten- 

 tion of observant men, and they have formed favourite illustrations 

 regarding the divisibility of matter. Several chapters in the 

 works of the celebrated Robert Boyle are devoted to this sub- 

 ject, and eminent men in all countries have speculated more or 

 less upon the extraordinary tenuity of the matter which is com- 

 petent to produce sensible effects upon our organs of smell. 

 Such a subject would of course in itself form a wide inquiry, 

 which it is quite out of my power to develope at present. I think, 

 however, that the apparatus which we have thus far made use of 

 enables us to deal with the question in a manner hitherto unat- 

 tainable. 



A number of dry aromatic plants f were obtained from Covent 

 Garden, the leaves and flowers of which were stuffed into glass 

 tubes 18 inches long and a quarter of an inch in diameter. By 

 means of my second air-pump, a current of dry air was first 

 passed over them for some minutes. They were then connected 

 with the experimental tube, which had its sources of heat arranged 

 as already described. The tube was first exhausted and the needle 

 brought to 0°, and dry air was then passed over the scented herbs 

 until the tube was filled. The consequent deflection was noted, 

 and from it the absorbent action of the odorous substance was 

 deduced. 



Thyme thus treated exercised thirty-three times the absorp- 

 tion of the air in which it was diffused. 



* I can hardly imagine the bands in the spectra of metallic compounds 

 to be produced by the vibration of the compound atom. All my experi- 

 ments show the vast influence of chemical union on the rate of oscillation ; 

 the metal itself and the compound of that metal could hardly, in my opinion, 

 oscillate alike. Hence I infer that decomposition has occurred when the 

 bright and constant spectral bands are seen. — June 20th. 



f I mean "dry" in the common acceptation of the term. They were 

 not green, but withered; doubtless, strictly speaking, they contained 

 aqueous vapour. 



Phil. Mag. S. 4. Vol 24 No. 1G2. Nov. 18G2. 2 A 



