OUR ASTRONOMICAL COLUMN. 
Comer 1g15sa (MELLisH).—A postcard from the 
Copenhagen Observatory gives the following ephemeris 
for comet 1915a, for Greenwich mean midnight :— 
RA. Decl. Log 4 
hm s. e ‘ 
Jan. 1 Bo St 0 eAGmel Peay 5: 0°7400 
9 Ble Sh Mass 42 59:8 - 4 67502 
17 6283520 42 518 o-7610 
25 BRIO), reas 42 41-2 0-726 
Feb. 2 MOG amas 42 29:0 0-7848 
10 BOuSs\. eeey 42 16:2 0-7975 
18 55 40 «se 42 3:7 08103 
26 RR oe abe 4I 52:0 08231 
Mar. 6 AVS5NSO.. <n cer eat 0-8357 
The estimated brightness of the comet is from the 
14th to the 15th magnitude. At the time of discovery, 
on February 10, 1915, the comet was of the 9th mag- 
nitude, and became visible to the naked eye during 
the following summer. 
Rotation AND Raptat VeELocity or N.G.C, 4594.— 
At the Mount Wilson Observatory an ingenious device 
has been employed by Mr. F. G. Pease to facilitate 
spectroscopic investigations of motion in faint nebulz 
(Proc. Nat. Acad, Sciences, vol, ii., p. 517). In this 
arrangement a silvered glass plate replaces the ordinary 
slit, and a slit is cut in the silver film at a place 
corresponding to each bright spot shown in a direct 
photograph of the nebula to be investigated, taken with 
the telescope to which the spectroscope is to be 
attached. The slits, of course, are chosen so as to 
prevent overlapping. For the comparison spectra 
another silvered plate is prepared, but with interrupted 
cuts, so that the central portions cover the parts pre- 
viously exposed to the nebula when the comparison 
spectra are impressed. In view of the long exposures 
required, great economy of time is thus secured. Mr. 
Pease has successfully employed this arrangement on 
the spiral nebula N.G.C. 4594, for which a total expo- 
‘sure of eighty hours was given. It was found possible 
to determine the velocity at five places in the nebula, 
and the values are represented by the equation 
y=—2-78x +1180, where y is velocity in km./sec. and 
x the distance from the nucleus in seconds of arcs. 
The radial velocity of the nebula is +1180 km./sec., 
while the rotational velocity at a point two minutes 
of arc from the nucleus is more than 330 km. Within 
the limits of error the rotational velocity increases 
linearly in passing from the nucleus, indicating that 
the nebula is rotating as a solid body, or, as seems 
more probable, that the material is moving in accord- 
ance with a law which will give a linear velocity curve, 
‘On certain suppositions the parallax would be 0:00013". 
In observations of such exceptional difficulty it is 
satisfactory to find a close accordance with the velocity 
+1100 km, given by Slipher for this nebula. 
Tue CooKEviLLe METEorITE.—A recently found iron 
meteorite, from Cookeville, Putnam County, Ten- 
nessee, is described by G. P. Merrill in the Proceed- 
ings of the U.S. National Museum (vol, li., p. 325). 
The meteorite is obviously very old, and so much 
oxidised that its original form is greatly obscured. 
The weight before cutting was 2132 grams. A cut 
surface shows an unusual feature in its very regular 
octahedral coarse crystallisation. Practically the entire 
mass is made up of broad kamacite bands 2 to 6 mm. 
in width, between which lie very thin plates of taenite. 
The total iron, of which nearly 20 per cent. occurs as 
oxides, is 81 per cent., while nickel amounts to 6 per 
cent. in the metallic form and 1 per cent. as oxide. 
Cobalt, phosphorus, sulphur, and carbon are present in 
small quantities. , 
NO. 2462, VoL. 98] 
NATURE 
[JANUARY 4, I917 
THE PYROGENESIS OF HYDROCARBONS. 
"Tae growing demand for low-boiling paraffins for 
use as fuel, and aromatic®*hydrocarbons for the 
manufacture of dyes and other materials, is directing 
increased attention to the possibilities of their syn- 
thetic production from natural sources, namely, the 
higher-boiling paraffins, coal and shale. It is well 
known that the superheating of the higher-boiling 
paraffins causes them to break up, or * crack,” into 
lower-boiling liquids, and the effect of temperature on 
the nature of the distillation products of coal has long 
been recognised. 
In a paper read before the Institution of Petroleum 
Technologists on November 21 the authors, Messrs. 
E. L. Lomax, A. E. Dunstan, and F. B. Thole, 
brought together not only a valuable bibliography of 
the literature on the subject (part i.), but, in the latter 
section (part ii.), also discussed in a comprehensive 
way the scientific aspects of this obviously complex 
process, and the various theories advanced by different 
workers in this field of inquiry. 
The earliest systematic study of ‘pyrogenic decom- 
position”’ (i.e. decomposition at high temperatures) of — 
hydrocarbons was initiated by Berthelot, who regarded 
the change as due either to simple polymerisation or 
condensation with loss of hydrogen. Moreover, each 
change being reversible, at a given temperature an 
equilibrium: was established between a complex series 
of decompositions, polymerisations, and condensations. 
Among the decomposition products acetylene was 
assumed to play an important rélé and to be the 
source more especially of 
Berthelot, in fact, obtained the latter by heating acety]- 
ene to a dull-red heat. Later observers have opposed 
this view on the ground that the presence of acetylene 
could not be demonstrated, and as an alternative 
suggested that hydrogen was first eliminated with the 
production of olefine and that the carbon chain was 
broken down atom by atom as methane, 
From a careful series of experiments on the thermal 
decomposition of methane, ethane, ethylene, and 
acetylene by Bone and Coward, it appeared that 
acetylene was the source of aromatic hydrocarbons, 
and was derived from the decomposition of ethylene. 
This polymerisation of acetylene takes place at 
600°-700°. At these high temperatures the ‘nascent 
radicals,” CH=, CH,=, and CH,—, are assumed to be 
formed, and either unite or, if hydrogen is present, 
undergo reduction. 
The authors of the present paper proceed to discuss 
the possible changes which may occur based on 
thermochemical data, and point out that the reactions 
which proceed by absorption of hydrogen are in the 
main exothermic, and the products, therefore, rela- 
tively stable, whereas cracking or decomposition is 
mainly the result of endothermic change. Now, 
according to the law of Le Chatelier, an exothermic 
synthesis will, at a high temperature, tend to be 
reversed, and the same is true of increased pressure, 
the tendency here being to bring about, under in- 
creased pressure, that change which diminishes the 
total volume of gaseous products. ; 
That the process of cracking is necessarily complex 
is easily realised when the nature of the material, and 
especially the temperature conditions, are considered; 
for an exothermic reaction which may occur at 
low temperatures may very well be replaced by an 
endothermic one at a higher temperature with a com- 
plete change in the nature of the products. Thus, at 
moderate temperatures up to 500° the tendency is for 
the formation of a mixture of paraffins and olefines, 
whilst at about 700° the effect is the generation of 
aromatic hydrocarbons. The effect of temperature 
has been well illustrated in the experiments made on 
! ethane, coal, and isoprene. 
aromatic hydrocarbons. — 
a 
