248 SCIENCE. 
normal temperatures to from —45° to — 
55° C. 
When magnesium light was used as the 
exciting agent, the resulting phospho- 
rescence did not become invisible until the 
substances were cooled to between —70° and 
—90° C. These temperature values might 
have been considerably different if other 
samples of the same phosphorescent mate- 
rials had been used. 
Other experiments by the undersigned 
were as follows: 
4, A test card coated with phosphorescent 
paint was placed under a color screen, com- 
posed of strips of red, yellow, green and 
blue, transparent celluloid. The paint was 
then exposed to sunlight, both at 20° C. and 
at the temperature of liquefied air (—188° 
C.). The phosphorescence produced at 
20° C. was more intense than at —188° C., 
but in the two cases the relative effects of 
the colored light appeared to be the same, 
blue light giving strong phosphorescence, 
while red light produced very little. 
5. A test card (‘A’) of phosphorescent 
paint was exposed to sunlight for two 
minutes, the first at 20° C., the second at 
—188° C. (in liquid air). The card was 
then taken into a dark room, where it be- 
came visibly phosphorescent when it had 
risen in temperature a few degrees above 
—188° C. When this card was warmed to 
20° C. it appeared equal in luminosity to a 
card ‘B’ that had been exposed one min- 
ute to the sun while in liquid air and then 
warmed to 20° C. Thus it appeared that 
the previous additional exposure to the sun 
of the paint on card ‘A’ of one minute at 
20° C. did not appreciably increase the in- 
tensity of its phosphorescence. 
The results of these experiments, as well 
as the observations of other writers men- 
tioned above, indicate that the principal 
effects of very low temperatures on phor- 
phorescence are as follows: 
A. That the reduction of the temperature 
[N. S. Vou. X. No. 243. 
of a phosphorescing substance is accom- 
panied by a corresponding decrease in the 
phosphorescent discharge. 
B. That very low temperatures cause 
phosphorescence to linger long enough to 
be readily observed in a number of sub- 
stances that are not visibly phosphorescent 
at normal and high temperatures. 
C. That the production of phosphores- 
’ cence in a phosphorescent substance is less 
when excitation occurs at low temperatures 
than when it takes place at high tempera- 
tures, other conditions being the same. 
Furthermore, it appears as if, for a cer- 
tain phosphorescent substance, different 
rates of phosphorescent discharge corre- 
spond to definite degrees of temperature, 
other conditions being the same, and that 
when an excited phosphorescent substance 
has been reduced to a very low tempera- 
ture (even to —200° C.), so as to show no 
phosphorescence, there is still some phos- 
phorescent activity present. In such a case, 
however, the eye is not sensitive enough to 
detect the existing feeble luminosity. 
A number of common substances show 
marked phosphorescence when reduced to 
the temperature of liquid air and then ex- 
posed to strong light; besides ivory and 
paper, already mentioned, are gum arabic, 
cotton wool, starch, white glue, celluloid 
and kid-skin. i 
The colors produced in the phosphores- 
cence of these substances were observed to 
be as follows: 
Gum arabic; decided light blue. 
Ivory; bluish white. (Some _ pieces 
greenish. ) 
Cotton wool ; bluish green, 
Starch ; yellowish green. 
Paper ; yellowish green. 
White glue; greenish yellow. 
Celluloid ; greenish yellow. 
Kid-skin (glove) ; decided green. 
All of these substances were placed in 
filtered liquid air, exposed to an arc light, 
