52 



KNOWLEDGE ♦ 



[Jan. 16, 1885. 



mysterious, one might say, if one could limit the pnssi- 

 liilities of future research) than the Mystery of Gravity. 

 It has been said, and truly said, that — 



Nature and Nature's laws lay hid in niglit. 

 God said, " Let Newton be," and all was light. 



All was indeed light where before men had been groping in 

 darkness. But outside the region where they had thus 

 been searchinij far vaster regions of darkness were revealed. 

 A veil was lifted, and one of Nature's mysteries was inter- 

 preted, but a more impenetrable veil was seen beyond, 

 which as yet no man has even hoped to lift. What greater 

 mystery can there be than this, that matter acts where it 

 is not 1 The sun on the earth and all his planets, the earth 

 on the moon, Jupiter and Saturn on their world■system^', 

 jiilanet on planet, star on star, nay, every particle of matter 

 on every other throughout infinity of space. And not only 

 so, but time seems annihilated as well as distance. For it 

 has been shown that unless the force of gravity traversed 

 distances with far greater velocity, nay, with many times 

 the velocity of light (187,000 miles per second), the wliole 

 mechanism of the solar system would long since have gone 

 wrong. Nor has it yet been shown how minute the time 

 intervals are in which practical infinities of distance are 

 traversed by this all-pervading attractive action. 



And now we find that not only is " inert " matter thus 

 intensely, one may say infinitely, energetic, as a cause of 

 motion, but that it is the real source of the light and heat, 

 which are in eflect the very life of the universe itself. 

 Within our own earth, the movements we call earthquakes, 

 as well as all such disturbances as volcanic eruptions, geysers, 

 and so forth, are all primarily due to the process of steady 

 contraction taking place under the action of terrestrial 

 .gravity, though, of course, the proximate cause of each such 

 disturbance is the heat generated during the process of 

 contraction. In each planet, no doubt, similar processes 

 are taking place, with greater or less energy, according as 

 a planet is younger or older. And now we see the 

 mighty mass of the sun, steadily by its gravitating energy 

 .generating heat, whose emission is, in fact, the very life of 

 the solar system. In other words, there resides in mere 

 matter, in what we have so long and so idly called inert 

 matter, the real source of every kind of movement, of 

 every kind of life within the universe itself For, what is 

 true of our sun is true of his fellow-suns, the stars, not 

 only of the thousands we see, but of the tens, the hundreds 

 of millions revealed by the telescope, and of the millions of 

 millions of galaxies of suns which doubtless exist beyond 

 the domain surveyed by our most powerful telescopes. 



But, turning from this stupendous, one may truly say 

 this awful mystei-y, which seems to present gravitation as 

 in a sense associated directly with the Great First Cause, 

 we note that there are many minor mysteries about the 

 theory that solar gravitation is the true source of solar 

 heat. 



In the fir.-t place, many find it difficult to understand 

 how the shrinkage of even solid matter, still less that of 

 vaporous matter, can lead to the generation of heat. Yet 

 in reality, to any one who rightly apprehends the principle 

 of the conservation of energy, it will be obvious that the 

 heat generated as the solar gravity draws inward any 

 portion of his envelope of vapours, and in so doing necessarily 

 diminishes the volume of that portion, must be exactly 

 the equivalent of the heat which would be required to 

 reverse the process, and so to restore by expansion that 

 portion of vaporous matter to its original volume. A 

 real difficulty arises for a moment, when we inquire why 

 the heat generated momentarily by the forces tending 

 to produce contraction is not momentarily employed 



in producing ecjuivalent counter-expansion. We must 

 remember, however, that a portion of the heat gene- 

 rated must necessarily be radiated away into space, simply 

 because it is exposed to the cold of space. One may com- 

 |iare the case to that of a pump so constructed that if all 

 the water raised remained in a certain vessel round the 

 place of exit, the weight of the water would serve as a 

 source of power to keep the pump working. In such a case, 

 were friction entirely gotten rid of, the pump would work 

 for ever. But if the vessel were perforated so that all the 

 time a portion of the water escaped, this would no longer 

 happen. The heat constantly generated by the enforced 

 process of solar contraction does in part escape ; a portion 

 continually undoes part of the work of contraction by 

 causing expansion, but the portion which, as we see and 

 feel, is continually escaping from the sun, causes a portion 

 of tlie contraction to remain uncompensated. Thus the 

 sun, as a whole, continues steadily contracting and steadily 

 emitting heat. Nor will he cease to contract until he 

 ceases to emit light and heat, or, in other words, until he 

 ceases to be the beneficent, light-giving centre of the solar 

 system. 



But the most perplexing mystery in connection with the 

 contraction-theory of the sun's heat is that the sun and the 

 earth seem to tell a diflerent story in regard to the amount 

 of heat which has been poured forth. Judging from the 

 sun's apparent size, it would seem as though not more than 

 twenty millions of years of so^.ar work, at the sun's present 

 rate of working, could possibly have been done ; for if he 

 had gathered in his mass from any distances however large, 

 that v/ould be the absolute maximum of energy represented 

 by the process of contraction to his present apparent size. 

 But the earth's crust seems to tell us of a much longer 

 period of sun work than this. If Dr. Croli, of Glasgow, 

 whose results Sir Charles Lyell accepted, has rightly esti- 

 mated the amount of work so done on the earth, it cannot 

 represent less than one hundred millions of years of sun 

 work. How can we explain the discrepancy t Professor 

 Young accepts the sun's evidence as it stands, making only 

 the proviso that if in past ages the sun was exposed 

 to some violent shock, as by collision with another sun, 

 more heat would have been generated. Dr. Croll takes 

 the earth's evidence alone, and considers that there must 

 have been such collision. For my own part, I consider it 

 clearly proved in other ways (and it come.s in well to 

 remove this particular difficulty) that the sun's real globe 

 is very much smaller than the globe we see. In other 

 words, the process of contraction has gone on further than, 

 judging from the sun's apparent size, we should suppose it 

 to have done, and therefore represents more sun work. 

 According to this view, however, the sun's future would 

 last many millions of years less than it would if his apparent 

 size is not far from his real size. But the limited allowance 

 of future work we should assign to him — a few millions of 

 years' work at the outside — is estimated on the assumption 

 that only such densities as we recognise in terrestrial sub- 

 stances can be attained within the sun's mass. It may 

 well be that under the conditions there existing, matter 

 may attain densities far greater than we find here. If so, 

 the sun's duration as the life-giving centre of a family of 

 worlds may be far greater than has been commonly 

 supposed. 



The frequenters of the Science Lectures at the Royal Victoria 

 Coffee Ilall, Waterloo Eoad, will be glad to learn that they were 

 I'esumcd on January 13, when Prof. II. G. Seeley, F.ll.S., lectured 

 on " More about the Wonders of the Sun " ; and on January 27, 

 Commander Cameron will relate " How I got from the East to 

 the West Coast of Africa." 



