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 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 precursor and inseparable associate of their luminous rays.

11. Absorption of Radiant Heat by Vapours and Odours.

We commenced the demonstrations brought forward in this Lecture by experiments on permanent gases, and we have now to turn our attention to the vapours 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 vapours allow the waves a comparatively free passage, in other cases the minutest bubble of vapour, introduced into the tube already employed for gases, 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