180 THE POPULAR SCIENCE MONTHLY. 



reproduction in certain media and at certain temperatures — are 

 scattered everywhere in the atmosphere. Interesting inquiries 

 into their distribution in air and water have been made by Dr. 

 Miquel at the Montsouris Observatory, Paris, and by Dr. Percy 

 Frankland in England. Dr. Frankland has found that the num- 

 ber present is much reduced in winter. Experiments made in in- 

 closed places, where there is little or no aerial motion, show the 

 number of suspended organisms to be very moderate ; but as soon 

 as any disturbance in the air occurs, from draughts or people 

 moving about, the number rapidly increases and may become 

 very great. Being slightly heavier than air, they have an invari- 

 able tendency to fall, and on that account collect on the surface 

 of water. Hence rivers, lakes, and ponds are constantly being 

 thus contaminated. 



Important points connected with dust of organic origin are its 

 inflammability and its liability to explode when mixed with air. 

 The property of explosiveness was forcibly illustrated in the de- 

 struction of six flour-mills by this cause in Minneapolis, Minn., in 

 May, 1878. Coal-dust in coal-mines is a cause of accident from 

 explosions which has been closely investigated in England, Ger- 

 many, and other mining countries. The subject was thoroughly 

 treated by Sir Frederick Abeel, in a paper on Accidents in Mines, 

 read before the Institution of Civil Engineers in 1888. 



Extremely fine particles of mineral dust may exist in the at- 

 mosphere, and do exist there more frequently than is generally 

 thought, while they escape detection by our senses. The author, 

 while making experiments on the Peak of Teneriffe, in 1878, found 

 the knife-edges of his balance so clogged with this invisible dust 

 that the balance refused to act. When wiped off, the dust col- 

 lected again in a few minutes, and it was only by continually 

 wiping it away that he was able to go on with his investiga- 

 tion. Prof. Piazzi Smyth, while on the Peak of Teneriffe, wit- 

 nessed strata of dust rising to a height of nearly a mile, reaching 

 out to the horizon in every direction, and so dense as to hide fre- 

 quently the neighboring hills. Prof. S. P. Langley, looking down 

 from the height of fifteen thousand feet on Mount Whitney, Cali- 

 fornia, into a region that had appeared clear from the valley below, 

 saw " a kind of level dust ocean, invisible from below, but whose 

 depth was six or seven thousand feet, as the upper portion only of 

 the opposite mountain-range rose clearly out of it." 



Dust storms are classified by Dr. Henry Cook, according to 

 their intensity, as atmospheric dust, dust columns, and dust storms. 

 Dr. Cook has observed in India that there are some days on which, 

 however hard and violently the wind may blow, no dust accom- 

 panies it, while on others every little puff of air or current of 

 wind forms or carries with it clouds of dust. If the wind which 



