19C4-5.] Dr J. Halm on the Nebular Hypothesis. 561 
caloric energy produced by the sun through contraction from infinity 
to the orbit of Mercury is little more than one per cent, of what he 
acquired afterwards through shrinkage to the present diameter. 
On the other hand, this loss may have been fully compensated by 
the impact of the meteoric cloud. Considering the enormous rise 
of temperature when this happened, it is not unlikely that the ring 
which probably first developed near the sun’s surface, where the 
destruction of orbital motion was greatest, through the heat de- 
veloped by the collisions, expanded and afterwards filled the whole 
space of the planetary system. 
But these are perhaps futile speculations which I will not pursue 
further, fearing that in this general outline already the hypothesis 
has stretched too far into the regions of uncontrolled imagination. 
Considered by itself, the theory would be of little value. But the 
fact, acknowledged by common consent, that collisions between 
stars and nebulae occur even now before our eyes in temporary 
stars, and that they are accompanied by phenomena which, judging 
from the spectroscopic evidence, point to the genesis of a rotating 
ring of nebular matter round the attracting body, is so suggestive 
of a similar course of events having been the cause of the rotation 
in our system, that I could not resist the temptation to venture 
upon speculative ground. Certainly no extraordinary gift of 
imagination is required to picture to ourselves the spectrum of the 
solar system under the initial conditions here assumed, with its 
expanding atmosphere of embryonal comets and the luminous ring 
of meteoric substance, the protoplasm of the future planets, and 
then to realise that this spectrum must have appeared to a distant 
observer in space as the typical spectrum of a new star. The 
“experimental” proof of the theory, afforded by the preceding 
examination of the spectroscopic evidence of temporary stars, is 
therefore encouraging, whatever may otherwise be urged against 
the superficial and highly incomplete treatment of so important a 
question in this communication. 
{Issued separately April 15, 1905.) 
PROC. ROY. SOC. EDIN. — YOL. XXV. 
36 
