Avaust 25, 1899. ] 
dowment of research laboratories. I have 
tried to make it clear that the physicist 
must avail himself of the powers and en- 
ergies set in play in the larger industrial 
enterprises, and finally that the field of pos- 
sible exploration in physics by experimental 
methods has its natural boundaries, outside 
of which our advances in knowledge must 
be derived from a study of celestial bodies. 
The riddle of gravitation is yet to be 
solved. This all-permeating force must 
be connected with other forcec and other 
properties of matter. It will be a delicate 
task, indeed, for the total attraction between 
very large masses closely adjacent, aside 
from the earth’s attraction, is very small. 
Scientific facts are of little value in them- 
selves. Their significance is their bearing 
upon other facts, enabling us to generalize 
and so to discover principles, just as the 
accurate measurement of the position of a 
star may be without value in itself, but in 
relation to other similar measurements of 
other stars may become the means of dis- 
covering their proper motions. We refine 
our instruments ; we render more trust- 
worthy our means of observation ; we ex- 
tend our range of experimental inquiry, 
and thus lay the foundation for the future 
work,with the full knowledge that, although 
our researches cannot extend beyond cer- 
tain limits, the field itself is, even within 
those limits, inexhaustible. 
Einv THomson. 
PHOSPHORESCENT SUBSTANCES AT LIQUID- 
AIR TEMPERATURES. 
A RECENT number of the Philosophical 
Magazine* contains a paper ‘On Phospho- 
rescence,’ by Herbert Jackson, which was 
delivered before the meeting of the British 
Association at Bristol, September 12, 1898, 
a portion of the paper being devoted to a 
review of the results obtained in researches 
relating to phosphorescent phenomena. It 
* Phil. Mag., Lon. 46, 281, p. 402, Sept., 1898. 
SCIENCE. 
245 
is evident from the paper that considerable 
investigation has been undertaken to ascer- 
tain the effects of high temperatures on 
phosphorescent substances, but that com- 
paratively little has been done towards 
learning the behavior of the latter at very 
low temperatures, such as are obtained by 
the use of liquefied air. It is stated, how- 
ever, in the paper referred to, that, ‘‘ Pro- 
fessor Dewar has shown that great reduction 
of the temperature will cause phospho- 
rescence to linger for a considerable time in 
many substances which had hitherto been 
considered as practically non-phospho- 
rescent.””? This in particular refers to the 
phosphorescence produced in certain sub- 
stances when exposed to light while at 
a temperature near that of liquefied 
air. Ivory, paper, and various other ma- 
terials show phosphorescence under such 
conditions, but little or none at normal 
temperatures (20° C.). 
Professor Dewar has found also that 
when a phosphorescent substance is excited 
by light at a normal temperature and then 
immersed in liquefied air the phosphores- 
cent discharge is practically suspended, and 
continues so while the substance remains at 
the low temperature. August and Louis 
Lumiere have recently published a note in 
the Comptes rendus, CX XVIII., No. 9,1899, 
p. 549, ‘Influence des températures trés 
basses sur la phosphorescence,’ to which 
reference will be made presently. 
The results obtained in some experiments 
made by the writer on the effect of liquid- 
air temperatures on phosphorescent sub- 
stances are given below. These experi- 
ments were already completed when it was 
learned that the above-mentioned note in 
the Comptes rendus had been published. 
They were as follows : 
Balmain’s luminous paint, which is 
strongly phosphorescent at normal tem- 
peratures, was subjected to a very low tem- 
perature by the use of liquefied air, boiling 
