The phenomena of the conversion of liquid into gaseous water or aqueous vapour, and its properties in that condition, have been stated in the articles BOILING Or LIQUIDS; EBULLITION; EVAPORATION; STEAM ; VAPOUR ; and VAPOUR OPALESCENT; these of its reconversion, or condensation, into liquid water and ice, under several heads above referred to, and also in the articles DEW and RAIN. The evaporation of ice has been noticed 'under Show.
The absorptive power for heat of aqueous vapour has recently been examined by Professor Tyndall, in his researches on the absorption and radiation of heat by gases and vapours, (' Phil. Trans.' 1861); in which lie found that hydrogen, the two gases which are the essential con stituents; of the atmosphere, and atmospheric air itself, absorb respec tively about 0-3 per cent, of the calorific rays emanating from a copper surface coated with lamp-black, heated by boiling water. " On a fair November day," be adds, " the aqueous vapour in the atmosphere produced fifteen times the absorption of the true air of the atmosphere. It is on rays emanating from a source of comparatively low temperature that this great absorptive energy is exerted ; hence the aqueous vapour of the atmosphere must act powerfully in intercepting terrestrial radiation ; its changes in quantity would produce corresponding change of climate. Subsequent researches must decide whether this vera causer, is competent to account for the climatal changes which geologic researches reveal." Proc. of Royal Soc.' vol. xi., pp. 101,102.
Under EVAPORATION, HYGROMETRY, and VAPOUR, an account has been given of Dr. Dalton's researches and views respecting the produc tion and tension of aqueous vapour and its relations to the atmosphere, which for many years have been almost universally accepted and relied upon. Meteorologists, accordingly, have been accustomed to separate the pressure of the aqueous vapour from the whole barometric pressure of the atmosphere, and thence to infer the pressure of the permanently elastic portion, or as it has been called, tho gaseous pressure, or the pressure of the dry air. Colonel Sykes, in a paper read
before the Royal Society some years since, and Lieut.-Col. Strachey in a recent communication ' On the Distribution of Aqueous Vapour in the Upper Parts of the Atmosphere,' have questioned both the truth of Dalten's hypothesis and the correctness of the application. A mathematical argument, showing the Incompatibility of the hypo thesis of a separate vapour atmosphere with the facts, will bo found In a paper by the late astronomer Bawl, translated in Taylor's 'Scientific Memoirs,' vol. ii. A more generally appreciable form has been given to this by Lieut.-Col. Strachey, who also has compared the results calculated from IMIton's hypothesis with the facts of the dis tribution of vapour in the atmosphere, as observed by Dr. Joseph D. Hooker, the late Mr. Webih, Colonel Sykes, and himself. From the entire investigation ho concludes that " The subtraction of the observed tension of vapour from the total barometrical pressure, in the hope of obtaining the simple gaseous pressure, must consequently be denounced as an absurdity ; and the barometrical pressure, thus corrected, as it is called, has no true meaning whatever." (' Proc. of Royal Soc.' vol. xi., pp. 182489.) On the other hand, the application of Doves method for obtaining the gaseous pressure, deduced from Dalton's hypothesis, in Major Oencral Sabinea paper on the Meteorology of Bombay,' published in the' Report of the British Association' for 1844, appears to have been successful in bringing out true results, which also verify the law assumed. It would appear that there is something important on the subject yet to be explained ; probably in respect of the relation of the observed phenomena which Dalton expressed by the hypothesis of gaseous substances being vacua, to each other, to their aggregate pressure when mingled, and the manner in which that is made up of their separate pressures, if such they have.