THE phenomenon noted by Mr. Poulton in NATURE (vol. xlvii. p. 608) is a matter of very common observation; except in the detail of the nail-heads it has been often noticed. The explanation is, I fancy, simpler than that suggested by Prof. Lodge. It is probably a simple case of sifting of air, as it passes by upward diffusion through the porous plaster, where its passage is not barred by contact of the plaster with the wood on the upper side. The plaster acts as the plaster of Paris plug does in the classical researches of Graham on the diffusion of gases, and as the plug of cotton-wool does in the common process of sterilising air in biological work. That warm air does stream up through a plaster-ceiling in this way is a matter of experience to every householder, when in the winter a bedroom over a sitting-room in which a fire is kept burning all day and a lamp or gas-flames for some hours in the evening, is always found to be drier and warmer than another room in the same house not so situated. We can scarcely classify dry wood and iron together as conductors of heat.
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IRVING, A. Soot-figures on Ceilings. Nature 48, 29 (1893). https://doi.org/10.1038/048029c0
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