THE DESEHTS of AFRICA AND ASIA. 331 



tenehrosum. This phenomenon, -wliicli Las occapied for a long time the 

 attention of physicists and naturalists, whose opinions are as con- 

 tradictory as unsatisfactory, has been recently studied by Dr. Gustav 

 Helleman, who, in an elaborate work, gives not only a chronological 

 enumeration of all the observations published on that subject from 1854 

 to 1874, but represents them graphically on a most instructive map. The 

 critical analysis of these numerous materials has suggested to the 

 Prussian philosopher, conclusions on the origin of this curious pheno- 

 menon, thoroughly opposite to those hitherto deduced by all his pre- 

 decessors, including the leai'ned Ehrenberg. These conclusions are : that 

 the above-mentioned clouds of dust proceed from the "Western Sahara, and 

 even partly from the French Sahara, for Dr. Helleman informs us that, 

 having consulted the meteorological registers kept in the French stations 

 of Laguat, Tnggnrt, Wargla, etc., he ascertained numerous days marked 

 in those registers as particularly characterised by Sirocco winds, accom- 

 panied by an atmosphere rendered dusty by sand. 



Another observation in reference to the composition of sands trans- 

 ported by winds from various countries yielded equally remarkable 

 results to Gustav Tissandier. Examining microscopically the dust 

 which fell the 9th of October, 1879, at Boulogne- sur-Mer, and comparing 

 it with the sand of the Sahara, he found that both were exactly of the 

 same composition, only the last was of a coarser grain, and that the frag- 

 ments of cryptogamic plants were in both identical. Moreover, the 

 French physicist ascertained that when sand from tlie deserts of Sahara 

 and Gobi was thrown into water, only the coarser particles fell to the 

 bottom, and the water remained turbid in consequence of a certain 

 quantity of thin mud swimming in it ; this last, having been micro- 

 scopically examined, turned out to be very like certain earthy particles 

 contained in rain-water, which sometimes is so rich in organic substances 

 that such rains have been qualified as manure rains. The result 

 which Tissandier deduces from repeated experiences is, that ' the winds 

 bring about a real selection of the smallest and lightest bodies of the 

 desert, upraising in the air only the minutest particles, among which are 

 the vegetable remnants, so that the atmospheric movements may convey 

 a dust very rich in organic substances, even when they have been bor- 

 rowed from a sand containing little of them, and that only because the 

 selection has been applied to large masses.' This curious observation 

 may suggest the inference that, at least in certain places, the sand of 

 deserts is not so barren and hostile to vegetation as it at first appears. 

 At all events, the absence of any animal traces in the Sahara and Gobi 

 sands examined by M. Tissandier, proves most evidently their subaerial 

 origin, for had they been deposited in sea-water they would include 

 numerous microscopic shells of Rhizopoda, which are so abundant in sea 

 sand, that Alcide d'Orbigny counted in one ounce of sand gathered in the 

 Antilles Islands, more than three millions of them ; and similar facts may 

 be ascertained in Europe, where, among other littoral regions, the sand of 

 the Adriatic shores contains numberless quantities of Rhizopodous shells. 



Again, M. Tacchini attributes an African origin to the highly ferrugi- 

 nous dust which not long ago fell in many places in Italy. What seems 

 to indicate such an origin is the fact that numerous globules of magnetic 

 iron have been found along the shores of Algeria and Tunis. The Italian 

 astronomer observes, that this fact has a certain importance, for it would 

 prove that in some cases a terrestrial but not a cosmical origin may be 



