80 ABSORPTION OF FOOD-SALTS BY LITHOPHYTES. 



whirled into the air by the wind. Moreover, the quality of the dust will no doubt 

 be characteristically affected by the vicinity of areas of industry. One has only to 

 look at the sooty leaves and branches of trees in parks near manufactories to 

 convince oneself of the reality of this influence. But it would be quite erroneous to 

 suppose that the air in regions far from land that has been cultivated or otherwise 

 opened up is free from dust. It contains dust everywhere. There is dust in the air 

 of the extensive ice-fields of arctic regions and of high mountain glaciers, and there 

 is dust in the air of great forests and over the boundless sea. 



If the rays of the setting sun fall obliquely through a gap between two peaks in 

 a wood-clad mountain valley, sun-motes may be seen floating up and down and in 

 circles, just as they do in a room when the last rays before sunset fall through the 

 window. These motes are of course not usually visible, and they are moreover 

 much smaller than the particles of dust which are raised by the wind from roads 

 and then again deposited. Now, when rain falls, it takes the sun-motes from the 

 air and brings them down to earth, and the air is thus washed to a certain degree 

 of purity. This iiappens still more completely in the event of snow. The latter 

 acts not unlike a mass of gelatine used to purify cloudy liquids, its effect being to 

 drag down with it all the particles to which the turbidity is due, leaving the upper 

 part of the liquid quite clear. Similarly, falling snow-flakes filter the air; and, 

 mixed with fallen snow, there are accordingly innumerable particles of dust. 

 If afterwards the snow gradually melts, it dissolves some of the dust, which then 

 drains away into chinks and depressions; but a portion remains behind undissolved. 

 This portion is gradually consolidated, and then appears lying on the parts of the 

 snow that are still unmelted in the form of dark patches, streaks, and bands; often 

 also it forms a smeary graphitic covering so widely spreading over the last remnants 

 of melting snow that the latter resemble lumps of mud rather than snow. Accord- 

 ingly we find it everywhere — in regions cultivated and uncultivated, in tilled 

 lowlands and on high grassy plains above forest limits, where no tilled land is to be 

 seen in any direction, and lastly in arctic regions in the middle of glaciers several 

 miles across. 



All this snow dust is not invariably deposited as a result of the filtering of the 

 air by falling snow-flakes; an additional supply is brought by the winds which 

 blow across the snow-fields. It is not of rare occurrence in the Alps for snow- 

 fields to exhibit suddenly, after violent storms, an orange-red coloration. On closer 

 inspection one finds that the surface of the snow is strewn with a layer of powder, 

 infinitesimally fine and for the most part brick-red, which has been brought by the 

 gales. Investigation of this " meteoric dust " shows that it is composed chiefly of 

 minute fragments of ferruginous quartz, felspar, and various other minerals. 

 Mixed with these there are, however, sometimes remnants of organic bodies, such 

 as bits of dead insects, siliceous skeletons of diatoms, spores, pollen-grains, tiny 

 fragments of stems, leaves, and fruits, and the like. Once, after a south wind had 

 prevailed for several days, the snow-fields of the Solstein range near Innsbruck 

 were covered, at a height of from two to three thousand meters above the sea-level, 



