304 



RADIATION. 



iiicandescence lias been produced, may be regarded as generating synclironons 

 vibrations in the platinum on which they impinge ; but once these vibrations 

 attain a certain amplitude, the mutual jostling of the atoms would produce quicker 

 tremors, and the light-giving waves would follow as the necessary progeny of 

 the heat-giving vibrations. From the very brightness of the light of some of 

 the fixed stars we may infer the intensity of the dark radiation, which is the 

 preem'sor and inseparable associate of their luminous rays. 



XI. — ABSORPTION OF RADIANT HEAT BY VAPORS AND ODORS. 



We commenced the demonstrations brought forward in this lecture by experi- 

 ments on permanent gases, and we have now to turn our attention to the vapors 

 of volatile liquids. Here, as in the case of the gases, vast differences have been 

 proved to exist between various kinds of molecules, as regards their i^ower of 

 intercepting the calorific waves. While some vapors allow the waves a com- 

 paratively free passage, in other cases the minutest bubble of vapor, introduced 

 into the tube already employed for gases, causes a deflection of the magnetic 

 needle. Assuming the absorption etlccted by air at a pressure of one atmosphere 

 to be unity, the following are the absorptions effected by a series of vapors at a 

 pressure of -g'^jth of an atmosphere : 



Name of vapor. 



Absorption. 



Bisulphide of carbon 47 



Iodide of methyl 115 



Benzol 136 



Arnylene 32 1 



Name of vapor. Absorption. 



Sulphuric ether 440 



Formic ether 548 



Acetic ether 612 



Bisulphide of carbon is the most transparent vapor in this list, and acetic 

 ether the most opaque ; ^Y)^^' ^^ ^" atmosphere of the former, however, pro- 

 duces 47 times the effect of a whole atmosphere of air, while J^th of an atmo- 

 sphere of the latter juoduces 612 times the effect of a whole atmosphere of air. 

 Itcducing dry air to the pressure of the acetic ether here employed, and comparing 

 them then together, the quantity of wave motion intercepted by the latter would 

 be man}' thousand times that intercepted by the air. 



Any one of these vapors discharged in the free atmosphere, in front of a body 

 emitting obscure rays, intercepts more or less of the radiation. A similar effect 

 is jn-oduced l)y perfumes diffused in the air, though their attenuations is known 

 to be almost infinite. Carrying, for example, a current of dry air over bibulous 

 paper moistened by patchouli, the scent taken up by the current absorbs 30 times 

 the quantity of heat intercepted by the air which carries it; and yet patchouli 

 acts moie feebly on radiant heat tlian any other perfume yet examined. Here 

 fidlow the results obtained with various essential oils, the odor, in each case, 

 being carried by a current of dry air into the tube akeady employed for gases 

 and vapors : 



Name of perfume. 



Absorption. 



Patchouli 30 



Sandal wood 32 



Geranium 33 



Oil of cloves 34 



Otto of roses 37 



Bergarnot 44 



Neroli 47 



Lavender 60 



Lemon 65 



Name of perfume. Absorption. 



Portugal 67 



Thyme 68 



Rosemary 74 



Oil of laurel 80 



Camomile ff owers 87 



Cassia 109 



Spikenard 355 



Aniseed 372 



