96 The Microscope. 



served with tlie microscope, and that in a variety of 

 interesting ways. For instance, the formation of the 

 metallic crystals of silver may be thus viewed, and 

 few objects give greater pleasure. A solution of 

 nitrate of silver is laid on the slide, a few copper 

 filings are dropped upon it, and instantly each grain 

 of copper becomes the centre of a brilliant tree-like 

 crystal of silver, growing rapidly as one views it, till 

 it reaches and perhaps shoots across another branch, 

 and the whole field is soon covered with glittering 

 foliage. And this is interesting as a chemical experi- 

 ment, showing the effect of " affinity." The copper 

 robs the silver of its nitric acid, obliging the silver to 

 appear in the metallic state. 



Or we may simply watch the effect of crystalliza- 

 tion in a salt without further admixture. Let us place 

 a drop of muriate of ammonia, rather thinly spread out 

 upon the slide. In a few moments some needle-like 

 crystals, straight and keen, dart across the field, and 

 then minor needles dart from them at right angles. 

 Now some half-dozen large darts spring forward we 

 think of a charge of bayonets, when suddenly every 

 weapon turns into a spruce-fir tree, with branches of 

 unusual straightness. 



This object is not altered by polarization ; but salt- 

 petre polarizes well. A drop of the solution remains 

 inert for 'a while, scarcely visible on the black ground 

 which we have obtained by a particular arrangement 

 of the prisms. Suddenly at the edges of the drop we 

 notice a few bright little crystals, like small books seen 

 in perspective, floating just within the boundary. 



