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 spoonful 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be rapidly withdrawn, and put into a vessel filled with water, 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 ochreous 
color assumed by the reagents.—/dem. 
62. Optical amusements.—Pierce a card with a small hole, 
and holding it before a window or white wall, 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 
is 
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. 
he 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- 
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 im 
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 higbly colored, is formed by two fa- 
ees still more inclined. The same effects may be obtained 
