PAST CHANGES IN TEE UNIVERSE. 



351 



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 of the universe 

 are in motion among themselves in straight lines 

 (much in analogy to the molecules of a gas, as 

 far as regards character of motion) ; that, under 

 the collisions that occur, a general balance is 

 maintained in the quantity of heat, and general 

 distribution and state of aggregation of the mat- 

 ter ; that some stellar suns are in the act of 

 cooling down, while others are renovated by fresh 

 collisions, changes continually occurring ia parts 

 of the universe, but the whole remaining un- 

 changed 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 noto- 

 rious 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 collisions), 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 explanation of the mech- 

 anism of gravity that has received support by 

 competent judges. 



From the deduction that the stars are moving 

 in straight lines in various directions, the analo- 

 gy (as regards character of motion) with a gas 

 becomes natural and inevitable ; for dynamical 

 principles are independent of size or relative scale, 

 or it is the same as if we were dealing with mole- 

 cules observed to have a proper motion among 

 each other in various directions. The analogy of 

 a gas presents itself, therefore, rather as a ne- 

 cessary consequence, not as a speculation. 1 The 

 1 The relative velocity of the stellar masses (mean- 

 in? by this the relative time taken by them to traverse 

 a distance equal to their own diameters) is almost in- 

 definitely slow compared with the case of the mole- 

 cules of gases, which traverse a distance equal to 

 many millions of times their own diameters in a single 

 second. The absolute velocity of the stellar masses, 

 however (as far as observed), is very great compared 

 with that of the molecules of gases, and consequently 

 the heat developed at their encounters would be very 

 great. On account of the large multiple the distance 

 of the stellar masses is of their diameters, they would, 

 no doubt, traverse vast distances before an encounter, 

 or their " mean length of path " would be very great ; 

 and (considering the relative length of time a stellar 

 mass takes to traverse a distance equal to its own 

 diameter) an enormous epoch of time would in gen- 

 eral elapse between the encounters. 



degree of aggregation of the separate parts of the 

 universe moving among each other in straight 

 lines would therefore appear to be necessarily 

 dependent on the mean velocity of proper mo- 

 tion ; just asethe degree of aggregation of the 

 components of the molecules of a compound gas 

 depends on the mean velocity of their proper 

 motion. If the velocity of the compound mole- 

 cules of the gas be increased (which is synony- 

 mous with what is called " raising the tempera- 

 ture ") they begin to separate into their compo- 

 nents (or to become " dissociated "), and if the 

 molecules have a very high complexity the ex- 

 tent to which this separation occurs (i. e., the 

 final degree of aggregation) depends on the tem- 

 perature or on the velocity of the molecules. If 

 the velocity be very high (by a high "tempera- 

 ture") the molecules split up — "dissociate" — 

 into their ultimate components. By gradually 

 lowering the temperature (velocity) the converse 

 process may take place, or the components may 

 gradually aggregate together to form massive 

 molecules. This analogy is applicable also to the 

 larger scale parts of the universe moving in 

 straight lines among each other, since dynamical 

 principles are independent of scale. Such an ex- 

 cessive velocity of the component parts of the 

 universe (the stars) among each other is quite 

 conceivable, at which the whole would break up 

 into single molecules. By a lower velocity these 

 molecules collect together under the action of 

 gravity into states of aggregation, or groups, on 

 the same principle (and perhaps even much on 

 the same process) as the constituents of the mole- 

 cules of a compound gas group together under 

 "chemical" action. The range of "chemical" 

 action, like the range of "gravific" action, is 

 limited ; the constituents of the stellar masses, 

 which group together under the limited range of 

 "gravity," being in this respect comparable to 

 the constituents of the molecules of a compound 

 gas which group together under the limited range 

 of "chemical" action. Just as in the case of a 

 compound gas (on account of some molecules 

 possessing much higher velocities than others) 

 the components of two molecules frequently be- 

 come dissociated by a collision, and group to- 

 gether again in a different part of the gas ; so 

 in the universe, where some stars (no doubt for 

 analogous dynamical reasons) possess a higher 

 proper motion than others, we have complete dis- 

 integration by collision in some parts, aggregation 

 in others, continual change, but the mean aggre- 

 gation remaining unchanged. 



It may be observed that, in principle, in order 



