358 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



It will perhaps be urged as an objection that 

 we have no experience of bodies moving in space 

 with velocities approaching to anything like 400 

 or 600 miles per second. A little consideration 

 will, however, show that this is an objection 

 which can hardly be admitted, as we are not in 

 a position to be able to perceive bodies moving 

 with such velocities. No body moving at the 

 rate of 400 miles per second could remain as a 

 member of our solar system. Beyond our sys- 

 tem, the only bodies visible to us are the nebulae 

 and fixed stars, and they are visible because they 

 are luminous. But the fixed stars are beyond 

 doubt suns similar to our own ; and, if we assume 

 that the energy in the form of heat and light pos- 

 sessed by our sun has been derived from motion 

 in space, we are hardly warranted in denying 

 that the light and heat possessed by the stars 

 were derived from another source. It is true 

 that the motion of the stars in relation to one 

 another, or in relation to our system (and this is 

 the only motion known to us), is but trifling 

 in comparison to what we even witness in our 

 solar system. But this is what we ought, a pri- 

 ori, to expect ; for if their light and heat were 

 derived from motion in space, like that of our 

 sun, then, like the sun, they must have lost their 

 motion. In fact, they are suns, and lisible be- 

 cause they have lost their motion. Had not the 

 masses of which these suns were composed lost 

 their motion they would have been non-luminous, 

 and of course totally invisible to us. In short, 

 we only see in stellar space those bodies which, 

 by coming into collision, have lost their motion, 

 for it is the lost motion which renders them 

 luminous and visible. 1 



1 "When the foregoing theory of the origin of the sun's 

 heat was advanced, in 1808, I was not aware that a paper 

 on the " Physical Constitution of the Sun and Stars" had 

 been read before the Eoyal Society by Mr. G. Johnstone 

 Stoney, in which he suggested that the heat possessed by 

 the stars may have been derived from collisions with one 

 another. " If two stars," he says, " should be brought by 

 their proper motion very close, one of three things would 

 happen : Either they would pass quite clear of one an 

 other, in which case they would recede to the same im- 

 mense dis'ance asunder from which they had come; or 

 they would become so entangled with one another as to 

 emerge from the frightful conflagration which would en- 

 sue, as one star; or, thirdly, they would brush against one 

 another, but not to the extent of preventing the stars from 

 getting clear again." In the latter case he considers a 

 double star is formed. Mr. Stoney's paper, though read 

 iu 1867, was not published till 1869. 



Mr. Herbert Spencer, in his " First Principles " (pp. 

 532-535), has also directed attention to the fact that the 

 stars distributed through space must tend, under the in- 

 fluence of gravity, to concentrate and become locally aggre- 

 gated. Separate aggregations will be drawn toward one 



The formation of a sun by collision is an 

 event that would not be likely to escape observa- 

 tion if it occurred within the limits of visibility 

 in space. But such an event .must be of very rare 

 occurrence, or the number of stars visible would 

 be far greater than it is. The number of stars regis- 

 tered down to the seventh magnitude, inclusive, 

 is — according to Herschel — somewhere between 

 12,000 and 15,000, and this is all that can possi- 

 bly be seen by the naked eye. Now, if we sup- 

 pose each of them to shine like our sun for (say) 

 100,000,000 years, then one formed in every 

 *7,000 or 8,000 years would maintain the present 

 number undiminished. But this is the number 

 included in both hemispheres, so that the occur- 

 rence of an event of such unparalleled splendor 

 and magnificence as the formation of a star or 

 rather nebula — for this would be the form first 

 assumed — is what can only be expected to be 

 seen on our hemisphere once in about 15,000 

 years. 



The absence of any historical record of such 

 an event having ever occurred can therefore be 

 no evidence whatever against the theory. 



NOTE ON SIR WILLIAM THOMSON'S AR- 

 GUMENTS FOR THE AGE OF THE 

 EARTH. 



Sir William Thomson has endeavored to 

 prove the recent age of the earth by three well- 

 known arguments of a purely physical nature: 

 The first is based on the age of the sun's heat ; 

 the second, on the tidal retardation of the earth's 

 rotation; and the third, on the secular cooling 

 of the earth. 



Argument from the Aye of the Sufi's Peat. — 

 It will be obvious that, if what has already been 

 advanced in regard to the origin of the sun's 

 heat be correct, it will follow that the argument 

 for the recent age of the earth, based upon the 

 assumption that the sun could have derived its 

 store of heat only from the condensation of its 

 mass, must be wholly abandoned, and that, in so 

 far as this argument is concerned, there is no 

 known limit to the amount of heat which the sun 



another, and ultimately coalesce. The result will be that 

 the heat evolved by such collisions taking place under the 

 enormous velocities acquired by gravity must have the 

 effect of dissipating the matter of which they are com- 

 posed into the gaseous slate. 



Both Mr. Stoney and Mr. Spencer consider the motions 

 of the cosmic masses to be due wholly to gravity, but, aa 

 we have seen, gravity alone cannot account for the enor- 

 mous amount of energy originally possessed by the sua. 



