186 OUTDOOR STUDIES 
were composed of nothing else, and seemed 
like heaps of blanched iron filings. 
Interesting observations have been made on 
the relations between ice and snow. The dif- 
ference seems to lie only in the more or less 
compacted arrangement of the frozen particles. 
Water and air, each being transparent when 
separate, become opaque when intimately min- 
gled, the reason being that the inequalities of 
refraction break up and scatter every ray of 
light. Thus, clouds cast a shadow; so does 
steam; so does foam: and the same elements 
take a still denser texture when combined as 
snow. Every snowflake is permeated with 
minute airy chambers, among which the light 
is bewildered and lost; while from perfectly 
hard and transparent ice every trace of air dis- 
appears, and the transmission of light is un- 
broken. Yet that same ice becomes white and 
opaque when pulverized, its fragments being 
then intermingled with air again, — just as col- 
orless glass may be crushed into white powder. 
On the other hand, Professor Tyndall has con- 
verted slabs of snow tq ice by pressure, and has 
shown that every glacier begins as a snowdrift 
at its summit, and ends in a transparent ice 
cavern below. “ The blue blocks which span 
the sources of the Arveiron were once powdery 
snow upon the slopes of the Col du Géant.” 
