ABSORPTION OP WATER. 87 



standing with its base a whole night in water. He states the mean 

 minimum of water contained by the sand of the dunes one foot below 

 the surface, after a long drought, at two per cent. ; the maximum, 

 after a rainy month, at four per cent. At greater depths the quantity 

 is larger. The hygroscopicity of the sand of the coast of Jutland he 

 found to be 33 per cent., by measure, or 21 '5 by weight. The 

 annual precipitation on that coast is 27 inches, and, as the 

 evaporation is about the same, he argues that rain-water does not 

 penetrate far beneath the surface of the dunes, and concludes that 

 their humidity can be explained only by the evaporation from 

 below. Om Klit formationen, pp. 106-110. 



"In the dunes of Algeria water is so abundant that wells are 

 constantly dug in them at high points on their surface. They are 

 sunk to the depth of three or four metres only, and the water rises 

 to the height of a metre in them. Laurent. Memoire sur le Sahara, 

 pp. 11, 12, 13. 



" The same writer observes (p. 14), that the hollows on the dunes 

 are planted with palms which find moisture enough a little below the 

 surface. It would hence seem that the proposal to fix the dunes 

 which are supposed to threaten the Suez Canal, by planting the 

 maritime pine and other trees upon them, is not altogether so absurd 

 as it has been thought to be by some of those distinguished philan- 

 thropists of other nations who were distressed with fears that French 

 capitalists would lose the money they had invested in that great 

 undertaking. 



" Ponds of water are often found in the depressions between the 

 sand hills of the dune chains in the North American desert." 



I have had occasion, in the volume on " Keboisement in France," to 

 refer to certain experiments by Thurmann, in which cubes of different 

 minerals, thoroughly dried, weighing each 100 grammes, were im- 

 mersed in water for five minutes. He states that these gave the follow- 

 ing results : — Liassic triassic, compact Jurassic, liassic triassic, and 

 oolithic limestones, granite, serpentine, basalt, dolerites, trachytes, 

 &c, gave a mean absorption of - 50 grammes of water. Similar mine- 

 rals, including gneiss and compact marl schist somewhat disinteg- 

 rated and changed, gave a mean absorption of 1 - 50 grammes ; lime- 

 stone still further decomposed, ferruginous oolites of Mt. Jura, liassic 

 schists and grits from the Vosges, and eruptive rocks perceptibly 

 changed, a mean absorption of £ grammes ; variegated grits, green 

 coloured grits, calcareous chalks, gravelly clay, and sands, 7 grammes ; 

 and clays, Oxford marls, and kaolin, an absorption of from 10 to 30 

 grammes. 



