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POPULAR SCIENCE REVIEW. 
In a paper on the Physical Origin of Nebulce, Dr. Croll endeavours to 
fathom the origin of the sun’s heat. It must he ultimately derived either 
from gravitation or from motion in spaee : hut whereas the former source of 
energy could not have exceeded from 20 to 30 million years’ supply of heat 
at the present rate of radiation, the store derivable from motion in space 
dependent on the rate of that motion may conceivably have amounted to any 
assignable quantity. A mass equal to that of the sun, moving with a velocity 
of 476 miles per second, possesses energy sufficient to cover the present rate 
of the sun’s radiation for 50 million years. 
This motion of translation must be converted into molecular motion by 
collision. The only assumption the collision theory makes is the existence 
of matter and motion. Suppose two bodies, each one half the mass of the 
sun, approaching each other directly at the rate of 476 miles per second, 
their collision would transform the whole of the motion into heat, affording 
an amount sufficient to supply the present rate of radiation for 50 million 
years. Each pound of the mass would, by the stoppage of the motion, 
possess as much heat as would melt 90 tons of iron. The whole mass would 
be converted into an incandescent gas, with a temperature of which we can 
form no adequate conception — certainly more than 140,000 times that of the 
voltaic arc. 
Effect of change of pressure on the length of disruptive discharge in air . — 
Mr. J. E. H. Gordon has been studying this at distances ranging from 6 to 
30 inches by means of the same apparatus. This was a fine induction-coil, 
giving a spark of 17 inches long, worked by 10 cells of a Grove’s battery. 
The discharging tubes were 4 feet long, and 3 inches in diameter, with a 
stuffing box at one end in which a brass rod slid. The inner end of the rod 
was kept in the axis of the tube by three little glass arms on an ebonite 
collar. The whole was insulated on ebonite legs. The tubes were connected 
with an air pump by means of an insulating glass tube. The pressure was 
given by a U guage about 4 feet high, and the air dried by passing it through 
sulphuric acid. 
In the experiments one tube was left open to the ' atmosphere, and its 
discharging point placed at a standard distance from the ball. The other 
tube being nearly exhausted, experiments were begun at low pressure, a little 
air being let in between each observation. The discharging distance in the 
second was then adjusted to the shortest distance which caused the whole 
discharge to pass in the first tube. This being noted, the points of the 
second tube were again brought together until the whole discharge passed 
between them. The mean distance of the two observations was used. 
The results were : — 
1. From a pressure of 11 inches upwards the length of spark is inversely 
as the pressure. 
2. Accidental circumstances materially modify the length of spark. 
3. Below a pressure of 11 inches the spark is shorter than it should be by 
No. 1, the electro-motive force required to produce a spark of given length 
at low pressure being greater. 
This agrees with Sir W. Thomson’s experiments, from which it results 
that greater electro-motive force per unit length of air is required to produce 
a spark at short distances than at long. To this may now be added, at low 
pressure than at high. The cause seems to be unexplained. 
