OLD 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 atmosphere were 

 wonderful. The white peaks were draped with opales- 

 cent 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 appeared to-day, while 

 the succession of surprises experienced through their 

 changes were such as rarely fall to the lot even of a prac- 

 tised 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 

 being 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. Xow 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 arti- 

 ficially, their particles are found to be far coarser 



