468 
known spectroscopic investigations. They 
find that the spectrum of oxygen consists of 
six series of lines, two principal groups each 
with two subordinate groups, having har- 
monic relations to each other. The spec- 
trum of oxygen is thus analogous to that of 
helium. At one time it was supposed that 
this character of the spectrum might indi- 
cate that the substance examined was really 
a mixture of two different elements closely 
resembling each other in properties. Many 
elements, such as sodium and lithium, give 
only one series of lines. This hypothesis 
has, however, been recently abandoned. 
W. J. Humphreys next read a paper on 
‘Changes in the Wave-Lengths of the Lines 
of Emission Spectra of Elements.’ The 
changes examined were caused by increas- 
ing the pressure of the air surrounding the 
electric arc up to six or twelve atmospheres, 
which was found to produce a shift of the 
line towards the red end of the spectrum, 
the amount of the shift being nearly in di- 
rect proportion to the pressure, and never 
exceeding a tenth of an Angstrom unit for 
the pressure employed. The shifts were in 
general proportional to the wave-length, 
but different series of lines were differently 
shifted. Shifts of similar lines of different 
elements are said to be inversely as the ab- 
solute temperatures of the melting points of 
the substances, proportional approximately 
to the products of the coefficients of linear 
expansion of the solid and the cube root of 
the atomic volume. The effect is, there- 
fore, like the atomic volume, a periodic 
function of the atomic weight. It is con- 
sidered that some of these relations may be 
accidental, as most of the time was spent in 
measuring the lines and not in hunting 
after empirical relations. 
In making the measurements, the best 
spectroscopic equipment of the Johns Hop- 
kins laboratory was used. Besides a num- 
ber of eye observations, several hundred 
photographs were taken, and almost every 
SCIENCE. 
[N. S. Vou. VI. No. 143. 
known metallic element examined. The 
shifts due to pressure may be distinguished 
from those due to the Doppler effect, be- 
cause different lines of the same element 
are differently shifted, and the direction of 
the shift is always to the red. The largest 
shift observed would correspond to a veloc- 
ity of recession of some four miles a second. 
A communication from Professor Schuster, 
‘On the Constitution of the Electric Spark,’ 
described measurements of the velocity of 
translation of metallic particles in the 
spark. The velocity found was from 400 to 
2,000 metres per second. 
Professor §. P. Thompson showed some 
experiments on varieties of cathode rays, 
which he classified according to their 
powers of exciting X-rays, of producing 
fluorescence, and of being deflected or not 
by a magnet. The ordinary cathode rays 
after reflection from the anticathode were 
found to be still fluorescifiant and deflect- 
able, but to have lost their power of excit- 
ing X-rays. A third variety was produced 
by passing the cathode rays through a 
negatively electrified spiral or sieve, which 
changed the nature of the fluorescence and 
rendered them non-deflectable. A fourth 
variety was found in the funnels of a Holtz 
tube. Under the hurried conditions of the 
section meeting, it was not very easy to 
follow and observe these several effects. 
Among the other papers on Friday were 
two by Professor §. P. Thompson, on a 
‘Tangent Photometer,’ and on an ‘ Experi- 
ment with a Bundle of Glass Plates.’ There 
were also two important contributions from 
John Hopkins on the comparison of the 
thermometers used by Rowland in his de- 
termination of the mechanical equivalent of 
heat (1) with the Paris Hydrogen Scale, 
and (2) with the scale of one of Callendar 
and Griffiths’ platinum thermometers. The 
results of these experiments were to show 
that, when reduced to the same thermo- 
metric scale, the results of Rowland for the 
