380 SCIENTIFIC RECORD FOR 1884. 



because there are too few of them. This fog renders it very doublftil 

 if thelawannouuccd by Aitken in 1879 can be tJ^ue, according to which 

 aqueous vapor will not condense from the air without having s solid 

 nucleus of dust particles on which to accumulate. If into the vessel 

 full of filtered air a small stream of steam enters, then the first cloud 

 formation of fine fog soon disappears, but after a few minutes large 

 scattered raiu drops strike on the surface of the inclosure, and form a 

 rain without fog or cloud. But if we allow a very small quantity of 

 dusty air to enter the vessel, then immediately fine fog is ibrmed along 

 side of the larger rain drops. If the beam of sunlight falls upon this 

 mixture of rain and fog, then a yellow halo with reddish brown borders 

 is formed of the same color and magnitude as with ordinary lunar 

 halos. If now by lowering the pressure we produce a sudden fall in 

 temperature, there is formed a system of larger fog particles and a cor- 

 resi^onding system of larger diffraction rings without changing those 

 already formed. In order to realize a fine mist or fog or haze of i>$irticles 

 having a very uniform size we must evidently have the maximum quan- 

 tity of moisture in the vessel, or the most favorable mist and the most 

 perfect saturation. This is best attained by introducing quite warm 

 moist air and lowering the temperature rather slowly. In this way we 

 obtain a very intense development of color, and the amplitude of the dif- 

 fraction circle varies between 10° and 30"^ depending upon the size of the 

 particles while the intensity of the color depends upon the uniformity of 

 their size. By farther diminution of pressure and consequent cooling 

 and increase in the size of the particles remarkable changes in color take 

 place ; the variations in the colors depend in a very sensitive way upon 

 the degree of saturation and the rapidity of cooling. In this way Kiess- 

 ling has been able to re-produce so large a number of the appearances 

 observed in nature that there can scarcely be any doubt but that the 

 diffraction due to particles of aqueous vapor will be found to explain 

 nearly all our sky colors. (D. M. Z., i, p. 117.) 



385. Dr. von Danckelman has published an extensive memoir col- 

 lating the meteorological observations made on the coast .of southwest 

 Africa, and especially at Vivi on the Lower Congo. A summary of his 

 results with regard to the cloudiness is given by him in D. M. Z.^ i, p. 

 301. 



Von Danckelman calls attention to the fact that the great forest and 

 prairie fires of the Congo in central Africa throw up into the atmos- 

 phere such an enormous amount of fine dust, that we have here a source 

 of production of fine particles that annually throws great quantities 

 into our atmosijhere — quite as much or vastly more than could have come 

 from the Krakatoa eruption. [D. M. Z., i, p. 311.) 



386. [The corresponding i)rairie aud forest fires in America, giving 

 rise to our Indian summer haze, have long been known to produce a 

 copper-red tint in the sky and over the solar disk. This haze however 

 has rarely been known to spread fai: eastward over the Atlantic, aud 



