MICROSCOPES. 261 



maline, so that the condensed light of the mirror be thrown 

 upon the object through the tourmaline. This polarizes the 

 light received. Then, above the eye-piece, lay another plate 

 of tourmaline. This serves as the analysing plate for per- 

 ceiving the changes produced by the polarized light. Many 

 crystals so observed, as sulphate of potass, &c., exhibit the 

 most beautiful coloring, according as the upper plate of 

 tourmaline is turned round on its axis. Some crystals, as 

 nitrate of potass, exhibit these colors without the upper or 

 analysing plate. The lamina of the crystal itself, as is the 

 case with the mineral called iolite or dichroite, serve to po- 

 larize the light passing through it, and to make it visible. 

 Instead of the upper plate of tourmaline, an analysing prism 

 of Iceland spar may be used, which exhibits these colors of 

 polarized light with still greater softness and perfection. 



Now, simple as are these means of observation, so pre- 

 sented to our notice, they will serve very widely to extend 

 our range of philosophical inquiry. The small port-object 

 of the microscope becomes a very effective chemical labora- 

 tory, where phenomena connected with chemical action may 

 be examined with an accuracy quite impracticable in larger 

 masses ; and for the purpose of analysing unknown sub- 

 stances, innumerable experiments may be carried on in a 

 short time where the quantities are comparatively insignifi- 

 cant. A door is thus opened into the arcana of nature, 

 which the man of truly scientific mind has only to enter, and 

 he will be gratified by an intimate observation of phenomena, 

 connected even with results which are, on the great scale, 

 quite familiar to him, that he will view with unfeigned sur- 

 prise. A microscopic acquaintance with the minute aspect 

 of chemical changes will bring him into nearer intimacy with 

 substances that he has long known, and guide him to con- 

 clusions which have hitherto only worn the dark character 

 of conjecture. 



A few other similar experiments may be mentioned. 



1. Add sulphuric acid to common salt or muriate of soda ; 

 or to carbonate of ammonia. 



2. Add sulphuric acid to bichromate of potass or muriate 

 of soda. The result is crystals of sulphate of soda and 

 potass, with chloro-chromic acid. 



3. Add acetic acid to bichromate of potass. The crys- 

 tallization of the bichromate takes place in very beautiful 

 forms. 



