[3] Modern Painters, vol. iv.
[4] Scenery of Scotland.
[5] Modern Painters, vol. iv.
[6] "The Alpine Regions of Switzerland" (Deighton, Bell, & Co.), a most interesting book, especially for travellers.
[7] It has lately been proved that clouds can only form in air which contains dust, and that each little suspended particle of water contains a speck of dust or a tiny germ of some sort for its nucleus.
[8] Pressure also has an important influence, but was omitted above for the sake of simplicity.
[9] Ruskin, "Modern Painters."
[10] The Alpine Regions of Switzerland.
[11] Mountaineering in 1861 (Longman).
[12] Mr. R. S. Watson, in "The Alpine Journal," vol. i., p. 143.