ATR AND LIFE. 199 



been noticed to fall over an area of 2,500 square miles. In the United 

 States, a similar phenomenon has been observed. Prof. S. P. Langley, 

 during the ascent of Mount Whitney, noticed that the middle strata of 

 the atmosphere contained a large amount of red dust which was visible 

 from above the level of these strata, while below, from the x>lains, no 

 trace of it was detected by the eye. This dust had, perhaps, its source 

 in China. The Krakatoa volcanic dust remained many years in the 

 atmosphere and traveled many times entirely around oar planet. 



All this dust becomes easily perceptible to the naked eye, when we 

 lpok at a ray of light in a dark room. But in order to well ascertain 

 its origin, to know exactly what it is, microscope and aeroscope are 

 wanted. By means of these instruments a very interesting microcosm 

 is revealed. All sorts of particles are to be found in the air — small 

 desiccated animals, such as worms, rotifers, vibrios, infusoria, frag- 

 ments of insects, of wool, scales from the wings of butterflies, particles 

 of hair, feathers, vegetable fibers, spores of fungi, pollen grains, flour, 

 dust from the soil, and microbes. From our present standpoint many 

 of these particles are of but slight interest to us, although it is a curi- 

 ous fact that volcanic dust may remain for years in the atmosphere at 

 considerable altitudes, and travel around the earth with the winds, 

 inducing those curious phenomena of light and color at sunrise and 

 sunset which physicists and the public at large observed after the 

 Krakatoa eruption. It is also a very curious fact, well illustrated by 

 T. Aitken's investigations, that these particles are favorable to the pro- 

 duction of rain. Under certain circumstances they play the part of a 

 nucleus around which the vapor of the atmosphere condenses, and each 

 particle becomes then the central part of a drop of rain. 1 What is of 

 interest to us, from the biological point of view, is the presence of pollen 

 grains, which explains how an isolated female plant may bear fruit even 

 at a great distance from male plants of the same species; the presence 

 of spores of fungi, which favors the dispersal of species; the presence of 

 light seeds, which may be carried very far, and then fall to the ground, 

 and develop an individual in a region where the species was never seen 

 before. Again, the presence of microbes, to which we have previously 

 referred, and which explains how many diseases are carried far and wide 

 by the agency of the winds, such microbes being specially abundant in 

 cities and in the vicinity of dwellings. At the Montsouris Observatory in 

 Paris, M. Miquel finds between 30 and 770 microbes per cubic meter of 

 air, according to winds, seasons, etc. ; in the center of the town, Eue de 

 Kivoli, the air contains 5,500 per cubic meter ; in hospital wards, between 



'Such being the ease, in order to induce artificial rain, instead of trying to change 

 the state of the atmosphere by means of explosions it would seem more rational to 

 send dust into the heights. But at all events, the essential requisite is the presence 

 of vapor, and this feature seems to have been sadly neglected in recent experiments. 

 Nature provides rain by means of vapor and changes in temperature, not by explo- 

 sions which can hardly have any influence. 



