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Absorption Of Radiant Heat By Vapors And Odors

( Originally Published 1905 )

We commenced the demonstrations brought forward in this lecture by experiments 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 power of intercepting the calorific waves. While some vapors allow the waves a comparatively free passage, the faintest mixture of other vapors causes a deflection of the magnetic needle. Assuming the absorption effected 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 1/60th of an atmosphere:

Name of vapor Absorption

Bisulphide of carbon 47
Iodide of methyl 115
Benzol 136
Amylene 321
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; 1/60th of an atmosphere of the former, however, produces 47 times the effect of *a whole atmosphere of air, while 1/60th of an atmosphere of the latter produces 612 times the effect of a whole atmosphere of air. Reducing dry air to the pressure of the acetic ether here employed, and comparing them then together, the quantity of wave-motion intercepted by the ether would be many thousand times that intercepted by the air.

Any one of these vapors discharged into the free atmosphere, in front of a body emitting obscure rays, intercepts more or less .of the radiation. A similar effect is produced by perfumes diffused in the air, though their attenuation 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 cur-rent absorbs 30 times the quantity of heat intercepted by the air which carries it; and yet patchouli acts more feebly on radiant heat than any other perfume yet examined. Here follow the results obtained with various essential oils, the odor, in each case, being carried by a current of dry air into the tube already employed for gases and vapors:

Name of perfume Absorption

Patchouli 30
Sandal wood 32
Geranium 33
Oil of cloves 34
Otto of roses 37
Bergamot 44
Neroli 47
Lavender 60
Lemon 65
Portugal 67
Thyme 68
Rosemary 74
Oil of laurel 80
Camomile flowers 87
Cassia 109
Spikenard 355
Aniseed 372

Thus the absorption by a tube full of dry air being 1, that of the odor of patchouli diffused in it is 30, that of lavender 60, that of rosemary 74, while that of aniseed amounts to 372. It would be idle to speculate on the quantities of matter concerned in these actions.

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