On the Latent Heat contained in the aqueous vapour in the Atmosphere.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 2, 1869, Page 413

By J. A. Wilson (3.) “On the Latent Heat contained in the aqueous vapour in the Atmosphere,” by J. A. Wilson. * Sent to author for revision, 26th January, 1869; not yet returned, March 25. —ED. This paper had appeared in the form of letters in the “Daily Southern Cross,” of November 19, 1864, and of December 24, 1864, under the heading, “Remarks on Australian and New Zealand Climatology, relative to our droughts, rains, and hot winds.” In his introductory remarks, Mr. Wilson said that his opinions on the subject were almost identical with those laid down by Professor Tyndall, in his famous lectures on heat. Dr. Purchas said he had observed one kind of rain upon the leaves of plants, in very large drops overhanging the edges of the leaves, and he could not say what was the reason of it. Mr. Stewart said there might be a difference in the leaves; if there was any in the rain it would show in analysis, and might depend on the atmospheric state at the time, and it might have a different effect on different plants. Mr. Wilson's paper had been most admirably got up, and would require much time for discussion. With reference to steam, the latent heat depends on the pressure of the steam, and it was ascertained by the greater quantity of water that would be required to reduce the steam to a working temperature, say of 100 degrees, and this even though the pressure of steam may be reduced by expansion. Dr. Purchas said that we must recollect that steam at high pressure was a different thing from steam at low temperature. High-pressure steam dissolves silica in caustic soda, but low-pressure steam will not. As for the rain on the leaves, kind has nothing whatever to do with the leaf, it is a peculiar description of rain, almost always accompanied by sickness. When it happened he always observed there was something very peculiar about the condition of the atmosphere. Mr. Peacock said it was a very valuable paper that Mr. Wilson had just read, and would require consideration for some time. In reference to Mr. Stewart's remarks upon steam, he, Mr. Peacock, was of opinion that the latent heat was not solely due to the pressure. Mr. Stewart said his remarks had reference to steam produced in a partial vacuum, where it could not flash like gunpowder into vapour at once. Ice would not immediately dissolve when dropped into boiling water—latent heat requires some time to be reduced. It was latent not so much in a partial vacuum, and conversely much more in a high-pressure boiler. Mr. Wilson said Professor Tyndall gives latent heat as equivalent to concealed heat, and he speaks of potential heat, possible heat, possible energy, possible power, dynamical energy, which is temperature, etc. In lifting a weight, for instance, a certain quantity of heat was expended in the action of drawing the arm up. We might speak of the difference between two substances, and the atoms in that substance. Weight has a power—a possible power. And if we take water, heat is a potent power in that substance, keeping apart the molecules, and expanding it. When the potential energy passes off, the actual energy takes place. Steam, for instance, can be evaporated at 32o, as well as at any higher temperature. Captain Hutton said the paper was a most valuable one, and it was certainly a curious question how the rain drops were so large when they fell, and how they came to grow so rapidly in falling. After some further remarks the meeting adjourned. Fourth Meeting. August 16, 1869. T. B. Gillies, President, in the chair. The names of the following new members were announced:—Dr. W. W. Watling, Dr. B. C. Beale, Dr. S. H. Ford, Mr. F. H. Meinertzhagen. A list of donations to the Museum was read by the Secretary. Papers read:— (1.) “On the Geology of the North Head of Manukau Harbour,” by Captain Hutton, F. G. S. (See ante, p. 161.) The paper described the formation of the locality treated upon, which is composed chiefly of volcanic rocks. It contained an interesting

account of the bearing of the lava streams and trachytic dykes, and treated of the resemblance which a portion of the district bears to the formation upon which Shortland and Grahamstown are built.