OT.T) ALPINE JOTTINGS. 455 



which the sybarite of the city could neither imitate nor 

 share. 



We spent about an hour upon the warm gneiss-blocks 

 on the top. Veils of cloud screened us at intervals 

 from the sun, and then we felt the keenness of the air; 

 but in general we were cheered and comforted by the 

 solar light and warmth. The shiftings of the atmo- 

 sphere were wonderful. The white peaks were draped 

 with opalescent clouds which never lingered for two 

 consecutive minutes in the same position. Clouds differ 

 widely from each other in point of beauty, but I had 

 hardly ever seen them more beautiful than they ap- 

 peared to-day, while the succession of surprises expe- 

 rienced through their changes were such as rarely fall 

 to the lot even of a practised mountaineer. 



These clouds are for the most part produced by the 

 chilling of the air through its own expansion. When 

 thus chilled, the aqueous vapour diffused through it, 

 which is previously unseen, is precipitated in visible 

 particles. Every particle of the cloud has consumed 

 in its formation a little polyhedron of vapour, and a 

 moment's reflection will make it clear that the size of 

 the cloud-particles must depend, not only on the size 

 of the vapour polyhedron, but on the relation of the 

 density of the vapour to that of its liquid. If the 

 vapour were light and the liquid heavy, other things 

 I eing equal, the cloud particle would be smaller than 

 if the vapour were heavy and the liquid light. There 

 would evidently be more shrinkage in the one case 

 than in the other. Now there are various liquids whose 

 weight is not greater than that of water, while the 

 weight of their vapour, bulk for bulk, is five or six 

 times that of aqueous vapour. When those heavy 

 vapours are precipitated as clouds, which is easily done 

 artificially, their particles are found to be far coarser 



