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POPULAR SCIENCE REVIEW. 
calcium, attached by means of some resinous medium, like varnish. While 
the material is far from novel, its method of manufacture and consequent 
condition give it such intensity as has never been approached before. The 
light given out is a violet-blue, like that which Becquerel produced with 
arragonite. Some portions glow by phosphorescence much more brightly 
than others; evidently this difference depends upon small structural or 
molecular variations. If further advances be made in this direction wonder- 
ful results may accrue. Thus, if walls were painted with such a substance 
they would absorb light enough during the day to continue luminous all 
night, and render all sources of artificial light useless. The colouring of 
houses on the outside with a like material would also evidently obviate the 
need of street-lamps. He does not expect that this remarkable and eco- 
nomical source of light will displace gas and all other sources of artificial 
illumination, but this new form of the phosphorescent sulphide of calcium, 
made of the cheap materials sulphur and lime, is a truly wonderful sub- 
stance, which may well suggest strange possibilities for the future. In the 
cabinet of the Stevens Institute are numerous specimens of phosphorescent 
powders — sulphides of calcium, barium, and strontium — which represent the 
best products heretofore obtained. These, if exposed to strong sunlight or 
to an electric discharge, will glow for many minutes in the dark. One of 
these clocks, however, he found, would continue to glow with sufficient 
brightness to be visible across a room all night, and could be read at any 
time, if approached closely. After being shut up in a box for five days this 
clock was still visible in total darkness, when the eyes had been rendered 
sensitive by remaining in the dark for a few minutes. This clock-dial is also 
readily “ excited ” by lamplight or gaslight, or indeed by any source of light 
containing rays above the yellow of the spectrum. The light from a Bunsen 
burner with soda in the flame, if filtered through yellow glass, will not 
excite it, but the blue rays of the Bunsen-burner flame will. The cause of 
the action is believed to be somewhat as follows : — When light falls on 
certain bodies its vibrations cause molecular changes which are not perma- 
nent, but are only maintained by the action of the “ exciting” vibrations, 
somewhat as a mass of plastic substance can be kept in a soft condition by 
constant stirring. When the exciting cause is removed the molecules return 
to their normal positions, and in so doing set up vibrations which are the 
cause of light, very much as the solidifying of water evolves heat. Thus 
these bodies, when exposed to daylight, absorb as it were the light energy? 
and re-emit the same afterwards. The phosphorescent property of sulphide 
of calcium has been known since 1768, when Canton prepared it by heating 
together intensely for an hour three parts of calcined oyster-shells and one 
part of sulphur. Its properties in this relation have been elaborately studied 
by Becquerel, who published his researches in the “Ann ales de Chimie et de 
Physique,” and has also devoted a large part of the first volume of his book, 
“ La Lumiere,” to this subject. He found that by employing lime in dif- 
ferent forms, such as Iceland spar, marble, oyster-shells, arragonite, &c., pro- 
ducts emitting different colours bv phosphorescence, such as orange, yellow? 
green, blue, and violet, were obtained. 
Collimator Adjustment. — Mr. A. Schuster suggests to the Physical Society 
a simple means of adjusting the collimator of the spectroscope ; the ordinary 
