The great importance of dust. in tlme fonuation of cloud and rain has been especially elucidated he time humors of \Ir. Jolmn Aitken, of Falkirk, Scotland, who has shown that in the ordinary process of the fonuation of cloud or fog by cool ing air the condensation of the uwisture takes place first upon the particles of dust as nuclei; that, in fact, every p:u•ticle of dust collects moisture• anml that every particle of cloud or fog implies a nucleus of dust. On this fundamuental principle Aitken has teased the construction of his so-called d mist -enunter, which lu• has made quite portable, and which enables one to dete*• mine with close proxinmity the relative and the absolute dustiness of any sample of air, lie has oracle such detenuinations in many puts of Eu rope and in a great variety of places. The dustiest air is found in the cities; thus, in Lon don, Edinburgh, and Paris there are from 80,000 to 210.000 particles per cubic centimeter (about the lifte(-nth part of a cubic inch). The air hav ing least dust, so fair a yet ohsened, is that of clue westcru lligh lit nds of Scot hand—nanmely, from 7600 down to 16 particles per cubic centimeter; the air of the Swiss nunnmtain-tops has a similar small content of dint. .\itken's investigations have expl:miued quautitatiych• that which was be• fore knocvu only qua lit atively—uanmelv. that Clue air nmost soothing to those who are troubled with delicate lungs is that which has the least dust. According to these results, the presence of dust makes it easier tor cooling air to form cloud or fog. Cloudy or foggy air must lie eou sidered as dnstless air in which the dust previous ly exist in_ is now replaced by mimmte floating globules of water. If, now, this dustless but air is still further cooled, its surphis aqueous vapor, which does nut easily condense upon tlue globules ahcady present• supersaturates the dust less air, thus hringing shout a condition of un stable equilibriums. This condition has been espe
cially investi,ated by llr. C. T. P. \\"ilson• of the Cavendislm Laboratory. ('anmbridge, England, who has shown Ilmat eondensation eventually lakes hut only after a considerable addi tional cooling and with violence. In this process latcnl he:mt is set free and phenomena produced whiclm imitate those observed in the interior of thunder-clmrmls froum c•lmich rain is fall irig. It would seenm plausible to conclude, therefore, that whereas dusty air is necessary for the fonnation of clouds. dustless air is necessary for the forma tion of rain.
When dust is very line and uniform in size, it gives rise to beautiful optical phenomena due to the diffraction and interference of light. such as the wonderful red. green, and yellow sunsets and sunrises, and the blue suns visible during the daytime in the yea•s 1883-85. These occurred in connection with the vapor-dust from the Kraka toa eruption: but similar phenomena have heel' observed on numerous other occasions after vol canic eruptions Among the important works on this subject may be mentioned C. G. Ehrenberg. out Infusoria ( Leipzig. 1838) and .1/ icro-yeutogy I85(1) .1. W. Bailey, numerous papers in the .4 1/11'7'kt/11 .Journal Of Srieli Cc, f 1'0111 183S to 1850 John ..,1itken, numerous papers in The Till to: fief/M/S Of the ROE/011 SOCiety of Edit/1)7(7•1/h. and his paper. Hbserrations of Atmospheric Dust. read before the Chicago Meteorological Con gress of 1893 (see United States Bureau Bulb tin No. 11, pp. 734-541: Carl Barns, on Cloudy Condensation (see bulletins and reports of the United States Weather Bureau) G. J. The Eruption of Krakatoa (London, Js88).