OF THE SOLAR SPECTRUM. 551 
fire; after cooling, these pieces are luminous in the dark after 
having been exposed to the light. It is also prepared by cal- 
cining in bones the sulphate of barytes reduced to powder. In 
every case it is the sulphuret obtained which possesses this 
faculty of being luminous under the influence of the solar rays. 
_ In general, all bodies which become phosphorescent by an 
elevation of temperature lose this property when they are kept 
a certain time at a temperature higher than the limit of tempera- 
ture at which they become phosphorescent. The greater num- 
ber regain this property by exposure to the solar light, others 
require being exposed to the influence of the light of an electric 
spark passing at a small distance from their surface. In the first 
_ class are the sulphurets of calcium, of barium, &c.; and in the 
~ second the crystallized minerals, as fluoride of lime, phosphate 
: of lime, &c. We are thus able to foresee the following results, 
which are confirmed by experiment: the sulphurets of calcium 
d of barium, prepared as above mentioned, become phospho- 
escent in a very high degree by insolation; if when they have 
“ceased to be luminous in the dark, which happens at the end of 
a quarter or half an hour, we raise their temperature, they cast 
a bright light during a few minutes, and are no more phospho- 
rescent by means of heat unless by a fresh exposure to the light. 
‘This exposure need be but of very short duration, since the ac- 
- tion of the light of an electric spark passing at some distance is 
sufficient to render them phosphorescent, as my father showed, 
and that the time which a spark lasts is = (v being the velo- 
city of electricity, or 700,000 leagues in a second, and d the 
tance of the two balls), a portion of time which does not ex- 
0°0000000000025. 
‘The solar rays exciting phosphorescence 1 in these sulphurets, 
: is necessary to examine in what parts of the spectrum the 
active rays which have been termed phosphorogenic are situated. 
Eo of the older natural philosophers, and especially Beccaria, 
said that the violet ray was the most apt and the red ray the least 
apt for exciting phosphorescence. Some experiments which M. 
iot and my father made on the phosphorogenic action of the 
electric spark, gave for result that the phosphorogenic rays differ 
the luminous rays. In reconsidering the action of the 
solar spectrum on phosphorescent bodies, I wished to observe if 
| im the spectrum of the phosphorogenic rays, which I shall call 
? 
| 
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