430 



SCIENCE. 



[Vol. XIII. No. 330 



It would seem as though our .views as to the condensation of 

 these fog-particles, and consequent liberation of latent heat, must 

 be very much modified. We are told that moist air, say, at 99 per 

 cent relative humidity, has different properties from saturated 

 air at 100 per cent. The first, by theory, has a diminution in tem- 

 perature of one degree in 1 88 feet vertical height ; while the second, 

 at 80°, has a diminution of one degree in 500 feet. It does not 

 seem possible for such enormous differences to exist in two masses 

 of moist air so near alike. It is supposed that dry air has a 

 capacity for molecules of vapor without change of volume, and 

 dependent entirely upon the temperature. For example : a cubic 

 foot of dry air weighs, at sea-level, 566 grams; the same saturated 

 will weigh 571 grams at 56°, and 576 grams at 77". If it weighs 

 571 grams at 77°, we say it is half saturated. It would seem as 

 though the arrangement of the molecules must be precisely the 

 same in the latter case as when the humidity is 99.9 per cent or 

 100 per cent, except that they are in a condition, in the moister air, 

 to unite more readily. It would be a great stretch of the imagina- 

 tion to consider that there is any marked difference in the condition 

 of the molecules at 99.9 per cent and at 100 per cent. 



Suppose we cool saturated air very slightly : all the molecules 

 cannot remain as vapor, but some of them coalesce, as do globules 

 of mercury when they touch each other. It would seem probable 

 that if, as many admit, rain is simply the coalescing of a very great 

 ■number of minute cloud-particles, we may extend the same action 

 a step farther back, and consider that the original molecules also 

 mechanically coalesce without setting free any latent heat. If this 

 be so, there certainly is not needed any dust-particle as a nucleus 

 for the mingling. We have now our air still saturated with vapor, 

 and at the same time full of, say, double molecules of vapor. If 

 we cool still more, the double molecules add others to themselves, 

 and we finally have our fog or fog-particles floating in saturated 

 air. 



It has been thought that these particles (spheres) are kept in 

 suspension very much as dust is, but this hypothesis seems unten- 

 able. Others have considered that each sphere is electrified, and 

 repels every other sphere ; and there is some color to this from the 

 fact that a vivid flash of lightning overhead is often followed by a 

 heavy and sudden downpour of rain. It seems probable that we 



can extend this hypothesis still farther back, and regard each mole- 

 cule as electrified. May not the coalescing of these molecules be 

 dependent upon their electrical state as much as or even more than 

 upon their cooling ? The principal point to be borne in mind is, 

 however, that the formation of fog and cloud is a purely mechan- 

 ical process, unaccompanied by the evolution of heat. A striking 

 proof of this was observed during an ascent of Greylock, in Massa- 

 chusetts, on Dec. 15, 1883. At the summit the wind was blowing 

 a whole gale, and the temperature was —7", with the air saturated 

 but perfectly clear. In a few minutes there were just barely per- 

 ceptible little white particles upon the overcoat. In a very short 

 time the aggregation of particles, absolutely invisible to the eye, 

 had become completely white. On the trees the particles had 

 massed nearly an inch thick. Another proof of this mechanical 

 aggregation is found in making observations with Regnault's dew- 

 point apparatus. It would be supposed that as the dew-point is 

 reached there would be a uniform deposit of molecules upon the 

 plate ; but this is not the case, as there are spaces between the 

 dew-particles. The effect is most noticeable when hoar-frost is 

 beautifully deposited in very marked lumps, at temperatures below 

 freezing. 



The cause of fog seems briefly as follows: i. It is essential that 

 there be no wind. I do not mean that the wind does not blow the 

 fog after it is formed, but there must be little or none while it is 

 forming. 2. The sky must be clear. We often notice a cloudless 

 sky after a fog is dissipated. On weather-maps, " fog " is entered 

 as " fair," for, though not a particle of sky is visible, yet it is almost 

 a certainty that the sky is clear. 3. The air must be saturated, or 

 nearly so. It is very surprising how rarely the last condition occurs 

 at inland stations. A relative humidity of 95 per cent has been 

 noted in the air, in which rain is falling, and had been falling con- 

 tinuously for seventeen hours. This condition almost always can 

 occur only to the south, south-east, or north-east of a storm. At 

 nightfall, whenever these conditions combine, there is a rapid radia- 

 tion from the earth to the sky, which speedily supersaturates the 

 overlying air ; and, after that, radiation from the upper surface of 

 the fog continues the process, and extends the fog upward until the 

 action ceases with the rising of the sun. H. A. Hazen. 



Washington, D.C., May 24. 



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TBB LAST NUUBEB EDITED BY ALLEN 

 THORNDIKE RICE. 



North American Review, 



JUISTE, 1889, 

 WITH A PORTRAIT OF MR. RICE. 



CONTENTS. 



Wealth Andrew Carnegie 



What is tjie Destiny of Canada ? 



Erastus Wiman 

 Unhappy Marriages in Fiction. . .Andrew Lang 

 How to Restore American Shipping, 



Nelson Dingley, Jr. 

 Last Days of the Steam Engine, 



R. H. Thurston 

 The Inevitable Surrender of Orthodoxy, 



The Rev. Minot J. Savage 



How a Census is Taken Carroll D. Wright 



The Mischievous Ice-Pitcher, 



Dr. William A. Hammond 

 Religious Value of Enthusiasm, 



William Booth, of the Salvation Army 

 Sir Arthur Sullivan and Piracy, 



Ale.Kander P. Browne 

 Why Am I a Quaker ? J. A. R. 



NOTES AND COMMENTS. 



Iconoclasm Necessary to Progress, 



Percy Douglas 



Illustrious, Seconds .Gertrude F. Atherton 



The Politics Nearest Home. ...Sylvester Baxter 



America for the Americans M. A. Westcott 



An Ignominious Destiny William Matthews 



A Plea for the Dialect Story, 



William E. Baldwin 



All Ne"ws<lealei*s. 



Fifty Cents. 



