198 AIR AND LIFE. 



as it moderates the heat of day and the cold of night, and acts as a 

 sort of regulator, by means of which some uniformity is established 

 under antagonistic and conflicting conditions, and in spite of contrary 

 influences. Quite certainly, if vapor did not exist, the physiology of 

 the animals and plants of the lowlands would be different, or they 

 would perish. 



The parts played by the numerous solid particles found in the atmos- 

 phere are as varied as is their nature. Physically pure air is a myth 

 and can only be obtained artificially, in laboratories, and when great 

 care is exercised. Even at the greatest heights, where micro-organisms 

 as well as vegetable or animal fragments are few and often totally want- 

 ing, mineral dust is always found. These particles are very small, to 

 be sure, and their origin varies ; some are of volcanic origin, and after 

 important eruptions, such as that of Krakatoa, volcanic particles are 

 very abundant in the atmosphere and may be years in settling or fall- 

 ing on land or sea; others are merely dust which the wind has swept 

 off the surface of the planet, and a large proportion consists of minute 

 fragments of aerolites which have fallen into the earth's sphere of 

 attraction from interplanetary space. 



Professor Newton has attempted to form some estimation of the 

 number of such aerolites, and he comes to the conclusion that our 

 atmosphere receives the enormous total of some 20,000,000 meteorites 

 per twenty-four hours, each of which is large enough to produce the 

 phenomenon known under the name of " shooting star." However 

 small these fragments may be — and yet in order to become visible 

 because of the heat evolved by friction against tbe atmosphere they can 

 not be so very minute — they certainly bring to our planet a consider- 

 able amount of foreign matter, a large proportion of which remains 

 some time suspended in the atmosphere before falling. In all places 

 where the requisite observations have been made, and where instru- 

 ments have been placed for collecting the mineral contents of the air, 

 there has been obtained an abundant harvest of meteoric particles, 

 easily recognizable by their form and structure, and the mud which 

 slowly accumulates at iiie bottom of the sea contains a large number of 

 these extra terrestrial bodies. As a matter of fact, mineral particles 

 of foreign source are constantly pouring through our atmosphere in 

 the form of a dry and invisible rain. A large amount of terrestrial dust 

 is also found in this rain. Von Richthofen speaks of the particular 

 aspect of the atmosphere in a part of China, where the sky is yellow 

 and opaque. When the wind comes from the direction of Central 

 Asia, all things are covered with a yellow dust which is brought 

 by the wind from vast regions whose soil is covered with a layer of 

 ochreous dust, which is driven to great distances over the Pacific. In 

 Australia, rains of a sort of red mud have been observed — rain made 

 into mud by the admixture of dust, the latter having been transported 

 by the wind and storm from considerable distances. Such a rain has 



