398 Intelligence and Miscellanies. 



stant the ebullition. Then add to the solution still warm, 

 ammonia till there is an excess. Stop the bottle and wait 

 until the precipitate is entirely formed. Then decant the li- 

 quid by means of a glass tube, wash the precipitate with wa- 

 ter previously boiled, and lastly, fill the bottle with warm 

 alcohol. 



When this protoxide is used, a small spoonfull of it is to 

 be rapidly withdrawn, and put into a vessel filled with water, 

 deprived of its air by boiling. Into this vessel the gas to be 

 examined must be passed. If it contain one part of oxygen 

 in a thousand, its presence will be indicated by the ochieous 

 color assumed by the reagents. — Idem. 



62. Optical anmsements. — Pierce a card with a small hole, 

 and holding it before a window or white wail, a pin being 

 held between the eye and the card will be seen on the other 

 side of the orifice inverted and enlarged. The reason of 

 this phenomenon as M. Lecat has observed, is, that the eye 

 sees only the image of the pin on the retina ; and since the 

 light which is arrested by the head of the pin, comes from 

 the lower part of the window or wall, while that which is 

 stopped by the lower end of the pin comes from the upper 

 part, the image must necessarily appear inverted relatively 

 to the object. 



The phenomena of the mirage may be completely imita- 

 ted, as Dr. Wollaston has shown, by directing one's observa- 

 tion to a distant object along an iron bar heated to redness, 

 or through a saline or saccharine solution, covered with al- 

 cohol. 



The following experiment, suggested by Dr. Brewster, ex- 

 plains" very agreeably the formation of halos : 



Put a few drops of a saturated solution of alum on a piece 

 of glass; it will rapidly crystallize in small octahedral plates, 

 scarcely visible to the naked eye. When this is held between 

 the eye and the sun, or a lamp, the eye being nearer the 

 smooth surface of the glass, three beautiful halos of light will 

 appear, at different distances from the luminous body. The 

 interior halo, which is the whitest, is formed by the images 

 refracted by two of the surfaces of the crystals, but little in- 

 clined to each other. The second halo, whose colors are 

 finer, is formed by two faces more inclined ; and the third, 

 which is very large, and highly colored, is formed by two fa- 

 ces still more inclined. The same effects may be obtained 



