550 E. BECQUEREL ON THE CONSTITUTION 
sure to the solar light, are so by the action of electric light, when — 
the discharge passes at a short distance from their surface ; more- 
over a great number of minerals become luminous, some when 
their temperature is raised, others when they are broken, struck, 
&c. It is probable that in these different circumstances, at the 
time of action of external agents, the electricities essential to the 
constitution of the bodies separate when there is any disturbance 
in the position of the molecules, then reunite more or less rapidly 
so as to give rise to the flashes which are observed. 
In this section, as we only treat of the action of the solar rays 
on phosphorescent bodies, nothing relative to bodies spontane- 
ously phosphorescent, such as wood, fish, &c., will be included. 
Amongst the substances which, after a short exposure to the 
solar light, become luminous in the dark, are Canton’s phospho- 
rus and the Bolognian stone; the one is the sulphuret of cal- 
cium, the other of barium. These two substances are the most — 
luminous by insolation. There are some others, such as the 
melted chloride of calcium (Homberg’s phosphorus), the melted 
nitrate of lime (Baldwin’s phosphorus*), the sulphuret of stron- 
tian, &c.; but these bodies are less sensible than the preceding 
ones. We shall only attend to the two first, because they are 
sufficient for the researches we propose to make, and for the 
deductions we wish to draw from them, 
The phosphorescent sulphuret of calcium is prepared in dif- 
ferent ways; it is obtained directly by calcining crystals of sul- 
phate of lime with powdered charcoal at a white red heat; this 
sulphuret thus prepared is luminous with a beautiful green light, 
after a previous exposure to the light. It may also be formed by 
calcining oystershells at a white red heat. In the same shells 
we thus obtain nearly all the prismatic colours. The two pre- 
dominant tints are the orange and green. But the intensity of 
the phosphorescence may be increased by calcining these shells 
afresh with a mixture of per-sulphuret of potassium. They then, 
by means of insolation, generally present a very intense green 
phosphorescence. 
The sulphuret of barium is also obtained with facility in the 
following manner: a paste is made with a mixture of sulphate 
of barytes and gum tragacanth, or with an organic mucilage, such 
as the white of egg, and this mixture is heated m a charcoal 
* The nitrate of lime is decomposed when heated. The phosphorescent sub 
stance is therefore a mixture of lizae, nitric oxide, and nitrous acid. 
