47 
NA TERE 
| SEPTEMBER 11, £yo2 
sphere, or in krypton or xenon, by more than one unit of wave- 
length on Angstrém’s scale, a quantity within the limit of 
probable error, Of the remainder, a good many agree to a like 
degree with argon lines, a very few with oxygen lines and still 
fewer with nitrogen lines ; the characteristic green auroral ray, 
which is not in the range of Humphreys’ photographs, also 
agrees within a small fraction of a unit of wave-length with one 
of the rays emitted by the most volatile atmospheric gas. 
Taking into account the Fraunhofer lines H, K and G, usually 
ascribed to calcium, there remain only fifty-five lines of the 
339, unaccounted for to the degree of probability indicated. 
Of ‘these considerably more than half are very weak lines 
which have not depicted themselves on more than one of 
the six films exposed, and extend but a very short distance 
into the sun’s atmosphere. There are, however, seven 
which are stronger lines, and reach to a considerable height 
above the sun’s rim, and all have depicted themselves on 
at least four of the six films.- If there be no considerable 
error in the wave-lengths assigned (and such is not likely 
to be the case), these lines may perhaps be due to some 
volatile element which may yet be discovered in our atmosphere. 
However that may be, the very great number of close coin- 
cidences between the auroral rays and those which are emitted 
under electric excitement by gases of our atmosphere almost 
constrains us to believe, what is indeed most probable on other 
grounds, that the sun’s coronal atmosphere is composed of the 
same substances as the earth’s, and that it is rendered luminous 
in the same way—namely, by electric discharges. This conclu- 
sion has plainly an important bearing on the explanation which 
should be given of the outburst of new stars and of the extra- 
“ordinary and rapid changes in their spectra. Moreover, leaving 
on one side the question whether gases ever become luminous by 
the direct action of heat, apart from such transfers of energy as 
occur in chemical change and electric disturbance, it demands a 
revision of the theories which attribute more permanent dif- 
ferences. between the spectra of different stars to differences of 
temperature, and a fuller consideration of the question whether 
they cannot with better reason be explained by differences in 
the electric conditions which prevail in the stellar atmosphere. 
If we turn to the question what is the cause of the electric 
discharges which are generally believed to occasion auroras, but 
of which little more has hitherto been known than that they are 
connected with sun-spots and solar eruptions, recent studies of 
electric discharges in high vacua, with which the names of 
Crookes, Rontgen, Lenard, and J. J. Thomson will always be 
associated, have opened the way for Arrhenius to suggest a 
definite and rational answer. He points out that the frequent 
disturbances which we know to occur in the sun must cause 
electric discharges in the sun’s atmosphere far exceeding any 
that occur in that of the earth. These will be attended with an 
ionisation of the gases, and the negative ions will stream away 
through the outer atmosphere of the sun into the interplanetary 
space, becoming, as Wilson has shown, nuclei of aggregation of 
condensable vapours and cosmic dust. The liquid and solid 
particles thus formed will be of various sizes; the larger will 
gravitate back to the sun, while those with diameters less 
than one and a half thousandths of a millimetre, but never- 
theless greater than a wave-length of light, will, in accord- 
ance with Clerk-Maxwell’s electromagnetic theory, be driven 
away from the sun by the incidence of the solar rays upon 
them, with velocities which may become enormous, until they 
meet other celestial bodies, or increase their dimensions by 
picking up more cosmic dust or diminish them by evapora- 
tion. The earth will catch its share of such particles on the 
side which is turned towards the sun, and its upper atmo- 
sphere will thereby become negatively electrified until the 
potential of the charge reaches such a point that a discharge 
occurs, which will be repeated as more charged particles reach 
the earth. This theory not only accounts for the auroral dis- 
charges, and the coincidence of their times of greatest frequency 
with those of the maxima of sunspots, but also for the minor 
maxima and minima. The vernal and autumnal maxima occur 
when the line through the earth and sun has its greatest in- 
clination to the solar equator, so that the earth is more directly 
exposed to the region of maximum of sunspots, while the twenty- 
six days period corresponds closely with the period of rotation 
of that part of the solar surface where faculz are most abundant. 
J. J. Thomson has pointed out, as a consequence of the 
Richardson observations, that negative ions will be constantly 
NO. 1715, VOL. 66] 
streaming from the sun merely regarded as a hot body, but this 
is not inconsistent with the supposition that there will be an 
excess of this emission in eruptions, and from the regions of 
faculz. Arrhenius’ theory accounts also, in a way which seems 
the most satisfactory hitherto enunciated, for the appearances 
presented by comets. The solid parts of these objects absorb 
the sun’s rays, and as they approach the sun become heated on 
the side turned towards him until the volatile substances 
frozen in or upon them are evaporated and diffused in 
the gaseous state in surrounding space, where they get 
cooled to the temperature of liquefaction and aggregated 
in drops about the negative ions. The larger of these drops 
gravitate towards the sun and form clouds of the coma about 
the head, while the smaller are driven by the incidence of the 
sun’s light upon them away from the sun and form the tail. 
The curvature of the tail depends, as Bredichin has shown, on 
the rate at which the particles are driven, whichin turn depends 
on the size and specific gravity of the particles, and these will 
vary with the density of the vapour from which they are formed 
and the frequency of the negative ions which collect them. In 
any case Arrhenius’ theory is a most suggestive one, not only 
-with reference to auroras and comets, and the solar corona and 
chromosphere, but also as to the constitution of the photosphere 
itself. 
Various Low-Temperature Researches. 
We may now summarise some of the results which have 
already been attained by low-temperature studies. In the first 
place, the great majority of chemical interactions are entirely 
suspended, but an element of such exceptional powers of com- 
bination as fluorine is still active at the temperature of liquid 
air. Whether solid fluorine and liquid hydrogen would interact 
no one can at present say. Bodies naturally become denser, but 
even a highly expansive substance like ice does not appear to 
reach the density of water at the lowest temperature. This is 
confirmatory of the view that the particles of matter under such 
conditions are nut packed in the closest possible way. The 
force of cohesion is greatly increased at low temperatures, as is 
shown by the additional stress required to rupture metallic wires. 
This fact is of interest in connection with two conflicting theories 
of matter. Lord Kelvin’s view is that the forces that hold 
together the particles of bodies may be accounted for without 
assuming any other agency than gravitation or any other law 
than the Newtonian. An opposite view is that the phenomena 
of the aggregation of molecules depend upon the molecular 
vibration as a physical cause. Hence, at the zero of absolute 
temperature, this vibrating energy being in complete abeyance, 
the phenomena of cohesion should cease to exist, and matter 
generally be reduced to an incoherent heap of cosmic dust. 
This second view receives no support from experiment. 
The photographic action of light is diminished at the tem- 
perature of liquid air to about 20 per cent. of its ordinary effi- 
ciency, and at the still lower temperature of liquid hydrogen 
only about 10 per cent. of the original sensitivity remains. At 
the temperature of liquid air or liquid hydrogen a large range of 
organic bodies and many inorganic ones acquire under exposure 
to violet light the property of phosphorescence. Such bodies 
glow faintly so long as they are kept cold, but become ex- 
ceedingly brilliant during the period when the temperature is 
rising. Even solid air is a phosphorescent body. All the 
alkaline earth sulphides which phosphoresce brilliantly at the 
ordinary temperature lose this property when cooled, to be 
revived on heating ; but such bodies in the first instance may 
be stimulated through the absorption of light at the lowest 
temperatures. Radio-active bodies, on the other hand, like 
radium, which are naturally self-lmminous, maintain this 
luminosity unimpaired at the very lowest temperatures, and are 
still capable of inducing phosphorescence in bodies like the 
platino-cyanides. Some crystals become for a time self- 
luminous when cooled in liquid air or hydrogen, owing to 
the induced electric stimulation causing discharges between 
the crystal molecules. This phenomenon is very pronounced 
with nitrate of uranium and some platino-cyanides. 
In conjunction with Prof. Fleming a long series of experi- 
ments was made on the electric and magnetic properties of 
bodies at low temperatures. The subjects that have been under 
investigation may be classified as follows : The Thermo-Electric 
Powers of Pure Metals ; the Magnetic Properties of Iron and 
Steel ; Dielectric Constants ; the Magnetic and Electric Con- 
stants of Liquid Oxygen; Magnetic Susceptibility. 
