ADDBESS. 21 



The difficulties to whicli I have referred have suggested to some 

 thinkers a different view of things, according to which it is not necessary 

 to suppose that one part of the system gravitationally supports another. 

 The whole may consist of a congeries of discrete bodies even if these 

 bodies be the ultimate molecules of matter. The planets may have been 

 formed by the gradual accretion of such discrete bodies. On the view 

 that the material of the condensing solar system consisted of separate 

 particles or masses, we have no longer the fluid pressure which is an 

 essential part of Laplace's theory. Faye, in his theory of evolution from 

 meteorites, has to throw over this fundamental idea of the nebular 

 hypothesis, and he formulates instead a different succession of events in 

 which the outer planets were formed last ; a theory which has difficulties 

 of its own. 



Professor George Darwin has recently shown, from an investigation 

 of the mechanical conditions of a swarm of meteorites, that on certain 

 assumptions a meteoric swarm might behave as a coarse gas, and in this 

 way bring back the fluid pressure exercised by one part of the system on 

 the other, which is required by Laplace's theory. One chief assumption 

 consists in supposing that such inelastic bodies as meteoric stones might 

 attain the effective elasticity of a high order which is necessary to the 

 theory through the sudden volatilisation of a part of their mass at an 

 encounter, by which what is virtually a violent explosive is introduced 

 between the two colliding stones. Professor Darwin is careful to point 

 out that it must necessarily be obscure as to how a small mass of solid 

 matter can take up a very large amount of energy in a small fraction of a 

 second. 



Any direct indications from the heavens themselves, however slight, 

 are of so great value, that I should perhaps in this connection call atten- 

 tion to a recent remarkable photograph by Mr. Roberts of the great 

 nebula in Andromeda. On this plate we seem to have presented to us 

 some stage of cosmical evolution on a gigantic scale. The photograph 

 shows a sort of whirlpool disturbance of the luminous matter which is 

 distributed in a plane inclined to the line of sight, in which a series of 

 rings of bright matter separated by dark spaces, greatly foreshortened by 

 perspective, surround a large undefined central mass. The parallax of this 

 nebula has not been ascertained, but there can be little doubt that we are 

 looking upon a system very remote, and therefore of a magnitude great 

 beyond our power of adequate comprehension. The matter of this nebula, 

 in whatever state it may be, appears to be distributed, as in so many 

 other nebulae, in rings or spiral streams, and to suggest a stage in a suc- 

 cession of evolutional events not inconsistent with that which the nebular 

 hypothesis requires. To liken this object more directly to any particular 

 stage in the formation of the solar system would be ' to compare things 

 great with small,' and might be indeed to introduce a false analogy ; but 

 on the other hand, we should err through an excess of caution if we did 



