THE SKY. 137 



proportions, in alcohol, and carefully dropping his solu- 

 tion into a beaker of water, kept briskly stirred, he 

 was able to reduce the precipitate to an extremely fine 

 state of division. The particles of mastic can by no 

 means be imagined as forming bladders. Still, against 

 a dark ground black velvet, for example the water 

 that contains them shows a distinctly blue colour. The 

 bluish colour of many liquids is produced in a similar 

 manner. Thin milk is an example. Blue eyes are 

 also said to be simply turbid media. The rocks over 

 which glaciers pass are finely ground and pulverised by 

 the ice, or the stony emery imbedded in it; and the 

 river which issues from the snout of every glacier is 

 laden with suspended matter. When such glacier water 

 is placed in a tall glass jar, and the heavier particles 

 are permitted to subside, the liquid column, when 

 viewed against a dark background, has a decidedly 

 bluish tinge. The exceptional blueness of the Lake of 

 Geneva, which is fed with glacier water, may be due, 

 in part, to particles small enough to remain suspended 

 long after their larger and heavier companions have 

 sunk to the bottom of the lake. 



We need not, however, resort to water for the pro- 

 duction of the colour. We can liberate, in air, parti- 

 cles of a size capable of producing a blue as deep and 

 pure as the azure of the firmament. In fact, artificial 

 skies may be thus generated, which prove their brother- 

 hood with the natural sky by exhibiting all its phe- 

 nomena. There are certain chemical compounds ag- 

 gregates of molecules the constituent atoms of which 

 are readily shaken asunder by the impact of special 

 waves of light. Probably, if not certainly, the atoms 

 and the waves are so related to each other, as regards 

 vibrating period, that the wave-motion can accumu- 

 late until it becomes disruptive. A great number of 



