5° 



HARDWICKE'S SCIENCE-GOSSIP. 



tinual loss of heat. Besides this, two sets of forces 

 are in connection with each member of the system 

 struggling for the mastery — one set tending to drive 

 the planets farther and farther away from the centre 

 of the system, the other set tending to draw them 

 towards the centre. Hitherto, probably the first set 

 of circumstances has had the advantage, and there is 

 little reason to doubt that all the planetary orbs, both 

 primary and secondary, are somewhat larger now than 

 they originally were. It has been proved that the 

 tidal wave, caused by the attraction of the moon, has 

 caused the earth to revolve more slowly by one 

 eighty-fourth of a second than it did at the beginning 

 of the Christian Era. The rotatory momentum thus 

 lost by the earth is not destroyed, but is used up in 

 driving the moon further from the earth, and the 

 more slowly our planet rotates, the further the moon 

 retires from it. A similar relation holds good in the 

 case of the planets and the sun. As between the 

 earth and the moon, to take one example, the 

 ultimate result must be to lengthen our day until it 

 corresponds with a lunar month, so that the earth 

 and moon will move in relation to each other as if 

 joined together by a rigid rod. While this is going 

 on, there is also an opposing set of circumstances at 

 work. We know there is no such thing as absolute 

 space. The existence of some form of matter is 

 absolutely necessary for the transmission of light and 

 other forms of radiance, and this interplanetary 

 matter offers resistance to the motion of the planets 

 which must tend to diminish their velocities. This 

 loss of velocity, if continued, would ultimately 

 bring all the planets into the sun, one after another, 

 beginning with Mercury and ending with Neptune. 

 It has been calculated that the earth would attain a 

 velocity of 379 miles per second immediately before 

 striking the sun, and that the heat generated by the 

 collision of the earth and sun alone, would produce a 

 temperature of nearly 9,000,000° F., so that after 

 the reunion of all the planets with the sun, the next 

 stage would be the dissipation of the whole mass into 

 an intensely hot nebula, probably not so large as the 

 original nebula, owing to the dissipation of energy 

 by means of the long-continued radiation of heat 

 from the solar system into space. To go back now 

 for a moment to the beginning, we may fairly surmise 

 that the existence of an intensely hot nebula was due 

 to some previous collision of cosmical bodies. 



Although the nebular hypothesis has not at present 

 been universally accepted, I am aware of only two 

 rival theories. One of these, published about 

 three years ago in Germany by two different men, 

 sought to account for the formation of the solar 

 system by a series of collisions, each collision 

 accounting for the formation of a planet. Any 

 number of collisions might be admitted, but it seems 

 utterly impossible that such a series could produce 

 one uniform system such as we have been con- 

 sidering, and I shall not dwell on this theory. 



Last January Mr. Croll, admitting the nebular 

 hypothesis, published what he terms the " Impact 

 Theory " to account for the formation of the nebula. 

 He assumes the existence of cold stellar masses in 

 motion. From our previous considerations there 

 should not be much difficulty in admitting the 

 presence of such masses in space. The nebuke 

 which we can see, point to the presence of con- 

 centrated matter which we could not see. Further, 

 it has already been pointed out that the planets 

 will themselves ultimately become dark stellar 

 masses. Should they then fall back upon the sun, 

 as we have supposed, a hot nebula would again 

 be formed by collision, but should other forces, at 

 present unappreciated, prevent their ultimate col- 

 lision, they would become stellar masses wandering 

 in space, which is what Mr. Croll starts with. To 

 account for the formation of our sun, he supposes two 

 bodies, each one-half of the mass of the sun, moving 

 towards each other with a velocity of 476 miles per 

 second — the velocity being due partly to original 

 motion and partly to the attraction of the two bodies 

 upon each other. The collision would shatter the 

 two bodies to pieces. The broken fragments now 

 forming one confused mass would rebound against 

 «ach other, breaking up into smaller fragments, and 

 flying off in all directions, continually striking 

 together and breaking up as they proceeded out- 

 wards. The heat of the collision would also generate 

 an enormous amount of incandescent gas, which 

 would act as a further expansive force. The heat 

 evolved would be mainly concentrated on the surface 

 layers of the broken blocks, and if we assume the 

 specific heat of the gas to be equal to that of air, it 

 would have a temperature of about 300,000,000 C, 

 or more than 140,000 times that of the voltaic arc. 

 The broken mass would, by the expansive force of 

 the generated gas, be dispersed in all directions, 

 breaking up into fragments smaller and smaller until 

 the fragments would become gradually converted into 

 the gaseous state and occupy a space as large as that 

 embraced in our solar system, and we should then 

 have a perfect nebula, intensely hot but not very 

 luminous. 



Laplace held that the solar nebula was intensely 

 hot. Since his time, however, it has become more 

 usual to consider the mass as being without heat, and 

 accounting for the presence of heat as the effect of 

 friction between the particles as the mass contracted 

 under the force of gravity. It has been calculated 

 that the heat generated thus by gravity would only 

 give 20,000,000 years as the age of the sun. This 

 time was thought to be much too short to account for 

 the various geological changes exhibited on the earth, 

 and Mr. Croll has accordingly formulated the 

 " Impact Theory," which, accounting for a hot 

 nebula, extends the age of the sun almost indefinitely, 

 according to the force of the original collision or 

 collisions. I must pass over the many interesting 



