246 
under atmospheric pressure. The sun and 
the electric arc were both used to excite 
phosphorescence in this substance. The 
temperature of the liquefied air employed, 
from experiments made with a platinum 
resistance thermometer, was determined to 
be approximately —-188° C. It varied sev- 
eral degrees during the experiments, on ac- 
‘count of changes in the composition of the 
liquefied air, due to ebullition. 
Balmain’s paint is a polysulphide of cal- 
cium mixed with varnish. The former is 
prepared by a secret process, but probably 
produced by mixing the powdered shells of 
certain shellfish with sulphur, and calcining 
these together at a high temperature in a 
closed crucible. 
A number of experiments were performed 
with Balmain’s paint at about 20 C. in 
order to ascertain the properties of this 
particular substance at ordinary tempera- 
tures. 
Test cards of the phosphorescent material 
were prepared for the experiments in the 
following manner : 
A sheet of cardboard was covered with 
dead black paper and a portion of it coated 
as evenly as possible with the luminous 
paint ; it was then cut into cards, each hav- 
ing a surface approximately 4x 5 centi- 
meters coated with paint. Test plates of 
thin sheet iron were also coated with the 
Same substance. 
The preliminary experiments, at normal 
temperatures (20° C.),showed: (1) that 
Balmain’s paint, exposed for a few seconds 
to the sun or the electric arc, gave bright 
violet phosphorescence when removed im- 
mediately away from strong light, and that 
after being in the dark for three hours its 
luminosity was so faint as to be hardly 
visible; (2) that a very slight elevation of 
temperature, a few degrees, caused a per- 
ceptible brightening of the phosphorescent 
surface, and that when the temperature of 
the substance was lowered, its luminosity 
SCIENCE. 
[N. 8S. Vou. X. No. 243. 
was lessened in a corresponding manner; 
(8) that the brightening of the phospho- 
rescent surface caused by the application of 
heat was merely a rapid discharge of the 
phosphorescent energy; (4) that a test 
card of luminous paint exposed to sunlight 
and then placed in darkness for three weeks 
showed phosphorescence when heated to 
about 800 degrees C.; (5) that the part of 
the solar spectrum producing phosphores- 
cence in the substance employed was prac- 
tically entirely that towards the violet end; 
(6) that the infra-red rays of the solar spec- 
trum falling on a phosphorescent surface 
rapidly discharged the phosphorescence. 
Those who have investigated this subject 
are no doubt familiar with these facts, but 
they are mentioned here for the purpose of 
comparison with the observations made at 
low temperatures. 
1. When a card covered with Balmain’s 
paint was exposed to strong sunlight, taken 
into a dark room, and then immersed in 
liquified air (about 188° below zero C.) 
its phosphorescence was so reduced as to 
appear perfectly destroyed. When, how- 
ever, this card was allowed to warm up 
gradually to the temperature of the room 
(20° C.) the phosphorescence again became 
active, being almost as bright as before im- 
mersion in liquefied air. 
2. A phosphorescent card treated in the 
manner just described, except rapidly 
warmed to the temperature of the room 
(from—188° C.) by immersion in water at 
20° C., was then compared with a card hav- 
ing had the same exposure to sunlight but 
not placed in liquid air. It was observed 
that the two cards presented little differ- 
ence in luminosity, showing that the tem- 
porary reduction of the temperature of the 
one placed in liquid air had not resulted in 
an appreciable change in its phosphorescent 
energy. The card which had been in the 
liquid air was slightly the brighter of the 
two. This was to be expected, because the 
