252 Chemical formulas of the Silicates. 
to that of a glass stainer’s kiln, without any change of color in 
the common colorless window glasses, while the same or similar 
specimens exposed to sunlight have been nearly all changed in 
a few months. 
Specimens exposed in hot water for a month, in doors and 
out of sunlight, experienced no change of color, while similar 
ones exposed during the same length of time in the bottom of 
a dish filled with two or three inches of water out of doors, and 
to the direct rays of the sun, experienced a decided change, 
though only about half as much as when directly exposed, out 
of the water. : 
ing convinced that air, moisture, and artificial heat do not 
make any change of color, our experiments indicate that the 
change is effected by the actinic rays of the sun alone. | 
F 
This actinic effect is cut off in some degree by every medium, 
by water as stated above, and even by clear glass, as a specimen 
exposed inside of a window or under another piece of thin col- 
orless glass shows only about one-half as much change, as that | 
exposed outside of the window or with no covering of glass 
over it. The amount cut off by colorless glass and by colored 
glass differs greatly with the difference of color. 
[To be concluded. ] 
———— 
Art. XXIX.—Crystallogenic and Crystallographic Contributions; 
by James D. DanA.—No. IV. Ona connection between YS | 
talline form and Chemical constitution, with some inferences there 
Jrom.—Supplement. 
I RETURN to the subject of my article published in the last 
number of this Journal in order to present a few additional 7 
marks with regard to the bearings of the views on chem! 
ee . 
fal cal formulas of the Silicates.—In the attempts to W™ 
the formulas of the Silicates on the new system there has been 
eg as they exist in these several states by 
ti 
a 
dreek letter to the 
