May 15, 1902] 
NATURE 
Sp) 
THE object of my letter to which Prof. Cox refers was to 
draw attention to certain statements made in recent accounts of 
Arrhenius’ theory which were disproved by Prof. Schwarzschild’s 
computations. I was fully aware at the time that Arrhenius 
himself had already arrived at the conclusion that, to accord with 
his theory, the particles in the tails must be assumed to be liquid 
or solid. This was the necessary result of his computations, 
which had convinced him that the diameters of the particles 
must be between o'r and 6x in order to satisfy Prof. Bredichin’s 
values for the repulsive forces observed in comets. But how 
does Arrhenius’ theory account for the presence of luminous 
vapours in the tail? Insome recent comets the typical spectrum 
of the hydrocarbons was traced by Prof. Vogel to the farthest 
end of their tails. The emission of Comet 1881 iv. (Schaeberle) 
was almost entirely gaseous, and in Comet 1882 ii. even the 
sodium vapour was observed in the brighter parts of its luminous 
appendage. How are these vapours carried into the extreme 
parts of the tail, since the analysis of Prof. Schwarzschild shows 
that the pressure of light is far too insignificant to exert a 
repulsion upon the molecules of a gas or vapour ? 
Prof. Cox assumes the evaporation on the side facing the sun 
to be caused by the ‘‘intense heat” to which the comet is 
exposed on its approach towards our luminary. He thus 
attempts the revival of an hypothesis now abandoned by 
astrophysicists. Astronomers will find it somewhat difficult to 
comprehend how, for instance, the famous comet of 1811—one 
of the most remarkable phenomena of last century—which 
never approached the sun to the distance of our planet, could 
have received so intense a heat-supply that the ‘‘ hydrogen of 
the hydrocarbons would boil off.” But apart from this, the 
spectroscope has now clearly demonstrated the luminosity of the 
cometary substance to be due to disruptive electric discharges 
at a low temperature. The assumption of an ‘intense heat” 
causing the evaporation on the side towards the sun receives no 
support from the spectroscopic evidence. Moreover, the misty 
film surrounding the nucleus, the so-called atmosphere or coma, 
is certainly of extreme tenuity, and the mass of the comet 
cannot but be immeasurably small. Hence the hydrostatic 
pressure opposed to the outpouring vapours must be extremely 
insignificant. This necessarily involves low boiling points, so 
that condensation can only take place at very low temperatures. 
Hence we require at the same time an intense heat to boil oft 
the hydrogen and an extremely low temperature to allow the 
condensation of the hydrocarbons into drops. 
But even suppose that in spite of the intense heat and the 
low hydrostatic pressure condensation does take place on the 
side towards the sun, and that drops of hydrocarbons with less 
than the ‘‘ critical” diameter are driven from the sun by the 
pressure of light. Can Prof. Cox demonstrate the possibility of 
these drops preserving their liquid state after having been 
launched into the vacuum of space? The permanent existence 
of drops of hydrocarbons in the tail is possible only under the 
condition that the space between the drops is saturated with 
hydrocarbon vapours. Here, then, we are again confronted 
with the question, How are these vapours carried into the tail, 
since the pressure of light has practically no repulsive power on 
the molecules of a gaseous substance? But if only drops, and 
no vapours, of hydrocarbon are repelled by the light-pressure, 
as Arrhenius assumes, what force prevents these drops from 
being instantaneously evaporated after once having departed 
from the outskirts of the comet’s atmosphere and having 
started on their journey through the vacuum of space? Ob- 
viously the assumed drops ought to retain their initial bulk 
throughout the whole length of the tail, z.e. through a distance 
of hundreds of thousands of miles, all the time unsurrounded 
by any vapour, the tension of which might counteract the 
inherent tendency of the liquid to assume the gaseous state. 
Such an hypothesis is plainly impossible. 
Prof. Cox mentions the possibility of solid particles being 
repelled by the light-pressure. He remarks that ‘‘ ultimately 
solid carbon is thrown out, finely divided as in an ordinary 
flame.” There is no objection to this assumption from- the 
physical point of view. But is it sufficient to explain the 
characteristic forms of the tails and their classification into 
several distinct types? What reason can be adduced for 
particles of dust assuming only such dimensions as would lead 
in all the comets to only three or four particular repulsive forces 
out of an infinite number of possible varieties? Why have, for 
instance, in the forty comets investigated by Prof. Bredichin, 
the particles never assumed such dimensions as would cor- 
NO. 1698, voL. 66] 
respond to types intermediate between Bredichin’s first and 
second? The explanation of Prof. Arrhenius is very un- 
satisfactory. He says :—‘* Wenn nun zufolge gewisser Umstande 
einige Tropfengrossen die gewOhnlichsten sind, so konnen die 
wohlbekannten, relativ scharf begrenzten Schweife von ver- 
schiedener Kriimmung entstehen.” No attempt is made in 
his paper to show what these ‘* certain circumstances ” are, nor 
why they should lead to the same types of tails in comets with 
widely different conditions of evaporation and condensation. 
The results of Bredichin seem to me indeed to be irreconcil- 
able with the present version of Arrhenius’ theory, which in no 
way explains the remarkable selection of repulsive forces dis- 
covered by the distinguished Russian astronomer. 
Another difficulty has been pointed out in my previous letter. 
It relates to the peculiar behaviour of the coma. In some 
comets a contraction of the coma has been observed on the 
approach of the comet towards perihelion, succeeded by an 
expansion after the perihelion had been passed. Thus the 
diameter of the coma of Encke’s comet in 1838 was found by 
Valz to have shrunk from~280,000 miles at the solar distance 
1°42 to only 3000 miles at perihelion. How isthis phenomenon 
to be explained by the pressure of sunlight ? In many instances 
(Comet 1862 ii., Respighi’s and Henry's comet among others) 
the coma retained its globular form while the tail spread out 
and assumed enormous dimensions. But the spectroscope has 
now demonstrated that no difference exists between the coma 
and the tail with regard to the physical and chemical constitu- 
tion of their materials. Hence the question remains still open 
why the pressure of light should repel the materials of the tail 
and yet at the same time leave the same materials in the coma 
entirely unaffected. 
My objection to the theory of Arrhenius refers to those parts 
where he introduces Maxwell’s pressure of light. Iam perfectly 
at one with the Swedish physicist and with Prof. J. J. Thomson 
regarding the important part probably played by the negative 
electrons emitted by the celestial bodies. But I fail to under- 
stand why the pressure of light should be required to account for 
the discharge of negative electrons into space. Physicists tell 
us that a hot body like our sun is most probably the source for 
an energetic emission of free electric atoms. We are, more- 
over, acquainted with the fact that these free electrons possess 
enormous velocities. The measurements of Wiechert have 
shown the velocity to be between one-fifth and one-third of that 
of light. Now if the heat of the sun is capable of splitting off 
the negative electron from its atom, a great number of these 
free electric atoms must be flung into space simply on account 
of their enormous kinetic energy. For no form of matter 
leaving the upper strata of the solar atmosphere with a velocity 
exceeding 600 kilometres per second can possibly return to the 
sun. Why, then, should the free negative electron, with more 
than one hundred times this critical velocity, still require a force 
such as the pressure of light to be propelled into the universe ? 
If we adopt Arrhenius’ idea, according to which the free 
electrons first condense ordinary matter around themselves near 
the solar surface and are afterwards driven off by the pressure 
of light on this bulk of matter, we must find it difficult to under- 
stand how in some authenticated cases the action of a solar 
outburst on the magnetic instruments could have been in- 
stantaneous (see Young, ‘‘The Sun”). Granting the highest 
possible repulsive action of light-pressure on small particles, 
the solar electrons would require at least sixteen hours to reach 
the surface of our planet. 
In my opinion, if we adopt the suggestion of Prof. J. J. 
Thomson that free negative electrons are probably emitted by 
the sun, a copious propagation of these infinitesimal corpuscles 
into space would be the obvious and necessary result of such an 
emission, even without the assumption of light-pressure. 
The train of reasoning ensuing from this hypothesis would 
lead in a most natural way to Zollner’s celebrated theory of 
comets. By the abundant presence of electrons, space has then 
to be considered as a negatively charged electric field acting 
upon the zovzsed cometary matter. From this point of view, 
Zollner’s theory—according to Newcomb, the one ‘‘ which on 
the whole most completely explains all the phenomena ”—would 
no longer ‘‘ lack the one thing needful to accept its reception,” 
namely, ‘‘the evidence that the sun acts as an electrified body.”’ 
The main conclusion I have arrived at after a careful study of 
the theory of Arrhenius amounts to this: that by abandoning 
the assumption of the pressure of light and by assuming the 
propagation of free negative electrons from the sun into space 
