THE SKY. 103 



impossible, task of forming bladders, when it passes into 

 the liquid condition. Let us examine the subject. Eau- 

 de-Cologne is prepared by dissolving aromatic gums or resins 

 in alcohol. Dropped into water, the scented liquid imme- 

 diately produces a white cloudiness, due to the precipitation 

 of the substances previously held in solution. The solid 

 particles are, however, comparatively gross; but by dimin- 

 ishing the quantity of the^dissolved gum, the precipitate 

 may be made to consist of extremely minute particles. 

 Briicke, for example, dissolved gum-mastic, in certain 

 proportions, in alcohol, and carefully dropping his solution 

 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 vel- 

 vet, for example the water that contains them shows a 

 distinctly blue color. The bluish color 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 pul- 

 verized 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 per- 

 mitted to subside, the liquid column, when viewed against a 

 dark background, has a decidedly bluish tinge. The excep- 

 tional 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 production 

 of the color. We can liberate, in air, particles 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 brotherhood with the natural 

 sky by exhibiting all its phenomena. There are certain 

 chemical compounds aggregates of molecules the con- 

 stituent 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 sub- 

 stances might be mentioned whose vapors, when mixed, with 



