be instantaneously illuminated by electric sparks, the separate vibration forms will be seen presenting half as many beads and flutes as are presented when the images are superposed through the employment of a continuous light. The lowest temperature at which the spheroidal condition can be produced varies with the nature of the heated surface, the liquid, and the temperature of the liquid when poured into the vessel. It is in virtue of this condition that Faraday found it possible to freeze mercury in a red hot vessel. When the metal is allowed to cool sufficiently, the liquid comes into contact with it, and is wholly or partially converted into vapour with explosive violence. In highly rarefied air water will assume the spheroidal condition at very low temperatures, in consequence of evaporation being accelerated by the diminution of pressure. Previous to the introduction of the molecular theory of gases many theories were proposed to explain the diffusion of aqueous vapour through the air. Halley supposed that vapour consisted of small hollow spherules or vesicles filled with an aura considerably lighter than air, which caused them to ascend like balloons, and Atwood followed his hypothesis. Even after the similarity of vapours to air and other so-called permanent gases had been fully recognized, the vesicular theory was still held in a modified f orm to explain the suspension of cloud and fog ; but in the case of very small drops the resistance of the air is sufficient to prevent the drops acquiring more than an extremely small velocity in consequence of their weight. Hooke supposed that air contains aqueous vapour in a state of chemical solution ; but this theory, like the preced ing, fails to explain evaporation in vacuo. De Saussure believed that water was first converted into vapour by the action of heat, and then absorbed by the air on account of a chemical affinity; while Halley, Leroy, and Franklin thought that the attraction of the air was instrumental in the first formation of vapour. The advocates of a still older theory maintained that aqueous vapour was a com bination of water particles with those of fire, which caused them to ascend, and that contrary winds blowing the particles of water together loosened the fire particles from them, thus allowing them to descend as rain. Desaguliers seems to have been the first to identify the nature of steam with that of aqueous vapour at ordinary temperatures, and to recognize the fact that steam is a transparent gas, while the cloud produced by a j t of steam is really condensed water. In a letter to the presi dent of the Royal Society (Phil. Tram., 1729, p. 6), Desaguliers maintained that the cause of vapour rising in the air is a force of repulsion I etween its particles, which separates them so far from each other as to render the vapour specifically lighter than air. The resistance offered by water to compression he accounts for by a similar repul sion. From some experiments with a steam engine he concluded that water in being converted into steam under ordinary atmospheric pressure expands to about 14,000 times its original volume instead of about 1650 times as it actually does. Shortly after the above-mentioned letter was written, Desaguliers, in " An Essay on the Cause of the Rise of Vapours and Exhalations in the Air " (Natural Pltilosophy, pt. ii.), attributed the repulsion between the particles of vapour to an electrical action, sup posing that the particles of water were first electrified from the air and then repelled by the air and by one another. In 1783 De Saussure published his Essais sur illi/r/ro- metrie, which give an account of many experiments executed on a great scale, and in some cases leading him to correct conclusions. By placing a known weight of dry potassic carbonate in a large glass balloon filled with air and satu rated with aqueous vapour, and finding the increase in the weight of the carbonate produced by absorption, he deter- 729 mined the amount of vapour originally present. By filling the balloon with dry air, and suspending in it a piece of wet linen, he determined the amount of the water which evapo rated from the loss of weight experienced by the linen. These experiments were repeated with the balloon filled with hydrogen and carbonic anhydride, and with mixtures of these gases, and both methods led to the same result, indicating that the amount of vapour was the same, if the temperature remained constant, whatever gas were present. The inferences he derived from his experiments at different temperatures were not, however, justifiable; nor is there any ground for his division of vapour into four classes, viz., pure elastic vapour, dissolved elastic vapour, vesicular vapour, and concrete vapour, the last of which really con sists of liquid drops. Deluc (Phil. Trans., 1792) enunciated the theory that the quantity of vapour which can exist in any space de pends only on the temperature, and is independent of the presence of any other vapour or gas with which it has no tendency to combine chemically, being always the same as if nothing but the vapour occupied the space ; and this he verified by placing his hygrometer with a little water under the receiver of an air-pump, and showing that the indica tions of the hygrometer were independent of the pressure of the air. Deluc was the first to propose that the hygrome- tric state of the air should be measured by the ratio of the amount of vapour existing in it to that required to saturate it at the temperature it possesses. A more convenient measure has been proposed by Balfour Stewart, viz., the quantity of vapour associated with the unit of mass of dry air. But it is to Dalton that we are chiefly indebted for a clear statement of the laws of evaporation. In his Meteoro logical Essays (17D3, p. 134) he states that " evaporation and the condensation of vapour are not the effects of chemi cal affinities, but aqueous vapour always exists as a fluid sui generis diffused amongst the rest of the aerial fluids." Thus water at 80 Fahr. is on the point of boiling under a pressure of 1 03 inches of mercury, and from this he con cludes that in the presence of dry air water at 80 Fahr. will evaporate "till the density of its vapour, considered abstractedly, becomes ^ th of what it is under a pressure of 30 inches, and its temperature 212." This statement, though inaccurate inasmuch as it takes no account of- the expansion of a given mass of steam at constant pressure when its temperature is raised from 80 Fahr. to 212 Fahr., yet shows that Dalton had discovered the true law of eva poration, and thoroughly understood its applications. If we substitute pressure for density, the statement becomes correct. Again, on page 201 of the Essays he states his conviction, as the result of experiments and observations, "That the vapour of water (and probably of most other liquids) exists at all temperatures in the atmosphere, ami is capable of bearing any known deyrce of cold without a total condensation, and that the vapour so existing is one and the same thing as steam or vapour of 212 or upivards. The idea, therefore, that vapour cannot exist iu the open atmosphere at a lower temperature than 212, unless chemically combined therewith, I consider as erroneous ; it has taken its rise from the supposition that air pressing upon rapoitr condenses the vapour equally with vapour pressing upon rapour, a supposition we have no right to assume, and which, 1 apprehend, will plainly appear to be contradictory to reason and unwarranted by facts; for when a particle of vapour exists between two particles of air, let their equal and opposite pressures upon it be what they may, they cannot bring it nearer to another particle of vapour, without which no condensation can take place, all other circum stances being the same; and it has never been proved that the vapour in a receiver from which all the air has been exhausted is precipitated upon the admission of perfectly dry air. Hence, then, we conclude, till the contrary can be proved, that the condensation of vajwur exposed to the common air does not in any way depend upon the pressure of the air." (The italics are Dal ton s.) In these remarks Dalton manifests a cleir appreciation of the true state of the case. In his experiments he aimed directly at the root of the
VIII. 92