322 LIFE IN THE ALPS. 



the view into stellar space. No trace of cloud is visible ; 

 and yet the substance from which clouds are made is, 

 at this moment, mixed copiously with the transparent 

 air. That substance is the vapour of water; and I take 

 this beautiful day as an illustration, to impress upon 

 you the fact that water-vapour is not a thing that can 

 be seen in the air. Were the atmosphere above and 

 around me at the present moment suddenly chilled, 

 visible clouds would be formed by the precipitation of 

 vapour now invisible. 



Some years ago, I stood upon the roof of the great 

 cathedral of Milan. The air over the plains of Lom- 

 bardy was then as pure and transparent as it is here 

 to-day. From the cathedral roof the snowy Alps are to 

 be seen; and on the occasion to which I refer, a light 

 wind blew towards them. When this air, so pure and 

 transparent as long as the sunny plains of Lombardy 

 were underneath to warm it, reached the cold Alps, and 

 was tilted up their sides, the aqueous vapour it con- 

 tained was precipitated into clouds of scowling black- 

 ness, laden with snow. 



If you pour cold water into a tumbler on a fine 

 summer day, a dimness will be immediately produced 

 by the conversion into water, on the outside surface of 

 the glass, of the aqueous vapour of the surrounding 

 air. Pushing the experiment still further, you may 

 fill a suitable vessel with a mixture of ice and salt, 

 which is colder than the coldest water. On the hot- 

 test day in summer, a thick fur of hoar frost is 

 thus readily produced on the chilled surface of the 

 vessel. 



The quantity of vapour which the atmosphere con- 

 tains varies from day to day. In England, north- 

 easterly winds bring us dry air, because the wind, before 

 reaching us, has passed over vast distances of dry land. 



