1878.J Past Changes in the Universe. 363 
masses (in a state of proper motion) arrived at on indepen- 
dent grounds by Mr. Croll is precisely the same conclusion 
that necessarily follows from the deduction that the range 
of gravity is limited , and therefore that the stars necessarily 
move among each other in straight lines, whereby, in the 
natural course of things, collisions are rendered inevitable. 
The eventuality of collisions among the stars is also treated 
of in a paper by Mr. Johnstone Stoney, “ On the Physical 
Constitution of the Sun and Stars ” (Proc. Roy. Soc., 
1868-69). 
The question of difficulty that remains would be to ex- 
plain how the colliding matter of the Universe is separated 
and prevented from aggregating together to an indefinite 
extent. The limit to the range of gravity and the limit to 
the intensity of gravity required by the physical theory 
would appear to have a bearing here. It may be observed 
that there are two opposing agencies, the expansive action 
of heat and the contractive action of gravity. By the con- 
tinued aggregation of matter the expansive action of heat 
increases, while the intensity of the contractive action of 
gravity attains a final limit. It would therefore appear 
reasonable to conclude that a balance would naturally set 
in, at which the expansive action of heat would compensate 
the contractive action of gravity, and thus a natural limit 
to the aggregation of matter would be reached. Thus the 
general result led up to by these conclusions would appear 
to be that the parts cf the Universe are in motion among 
themselves in straight lines (much in analogy to the mole- 
cules of a gas, as far as regards character of motion) ; that, 
under the collisions that occur, a general balance is main- 
tained in the quantity of heat, and general distribution and 
state of aggregation of the matter ; that some stellar suns 
are in the act of cooling down, while others are renovated 
by fresh collisions, changes continually occurring in parts 
of the Universe, but the whole remaining unchanged in 
character. That sudden developments of heat among the 
stars and (as it were) the flashing out of suns do occur at 
times, is a notorious fact in astronomy. It should be noted 
that the above fundamental deduction, that the parts of the 
Universe are moving in straight lines (which entails colli- 
sions), cannot be regarded as a mere speculation, but is a 
necessary inference following on the recognition that the 
range of gravity is limited , and on the fact that the stars 
are actually observed to possess proper motions in various 
directions. The deduction that the range of gravity is 
limited is itself a necessary consequence of the one 
