200 



NATURE 



\yune 27, 1889 



geological estimates as 300,000,000 years for the ' denu- 

 dation of the Weald ' ? Whether is it more probable 

 that the physical conditions of the sun's matter differ 1000 

 times more than dynamics compel us to suppose they 

 differ from those of matter in our laboratories ; or that a 

 stormy sea, with possibly channel tides of extreme violence, 

 should encroach on a chalk cliff 1000 times more rapidly 

 than Mr. Darwin's estimate of one inch per century?"* 



But granted that the geological evidence is against the 

 gravitation theory, it remains for us to see how Dr. 

 Croll's theory bears the strain put upon it when the 

 details of the evolutionary processes are inquired into. 



According to the impact theory of Dr. Croll, " meteorites 

 are but the fragments of sidereal masses which have 

 been shattered by collision" (p. 12). The result of such 

 a collision would be mainly to produce a gaseous mass, 

 but some of the exterior fragments would have velocities 

 sufficient to carry them beyond the influence of the central 

 mass. This view is obviously in direct contradiction to 

 the opinion held by Mr. Lockyer, who looks upon meteo- 

 rites as the parents, and not the children, of sidereal 

 systems. The explanation of the thumb-marks and the 

 heterogeneous structure of meteorites which has been 

 given by Mr. Lockyer (Proc. Roy. Soc, vol. xliii. p. 151) 

 would apply equally to Dr. Croll's view. 



Comets, according to the impact theory, have a similar 

 origin to meteorites. Dr. Croll apparently agreeing that 

 they are nothing more than swarms of meteorites. Those 

 with elliptic orbits probably had their origin in the colli- 

 sion which produced the nebula out of which the solar 

 system has been evolved, whilst those with parabolic 

 and hyperbolic orbits are probably the outcasts of other 

 systems. 



The first condition of a nebula, according to Dr. 

 Croll, is that in which it consists of broken fragments 

 scattered through a gaseous mass of excessively high 

 temperature. Mr. Lockyer's recent researches are con- 

 sistent with this view, as far as meteorites and interspaces 

 are concerned, but they point to the opposite conclusion 

 with regard to temperature. Mr. Lockyer's spectroscopic 

 work has shown that the highest temperature is in all 

 probability only reached by a nebulous mass after the 

 complete volatilization of all the meteorites composing it, 

 and he has shown that the intermediate stages which 

 should occur on this supposition are actually represented 

 amongst the stars, the stars of Group IL being amongst 

 these. This therefore furnishes a strong argument against 

 the high-temperature theory of nebulte. 



According to the impact theory of Dr. CrolL the 

 meteorites scattered through the gaseous mass have 

 nothing whatever to do with the luminosity ; whereas, 

 from Mr. Locker's point of view, the luminosity is in great 

 part, if not entirely, due to collisions between the meteo- 

 rites. Dr. Croll objects to this latter view because it 

 " does not appear to afford any rational explanation of 

 this banging about of the stones to and fro in all direc- 

 tions ; for, according to it, the only force available is 

 gravitation, and this can only produce merely a motion of 

 the materials towards the centre of the mass" (p. 20). 

 Dr. Croll has evidently given but little thought to this 

 theory, originally advanced by Prof Tait,for it is obvious 

 that all the meteorites would not lose all their momentum 



' Macinillan s Magazine, March 1862 ; reprinted in " Popular Lectures and 

 Addresses," p. 361 (Macmilian, 1889), 



by collisions during their first movements towards the 

 common centre of gravity. Those which escaped colhsion 

 would move on beyond the centre of gravity with con- 

 siderable velocities, and would continue to oscillate to 

 and fro until all their momentum was converted into heat 

 by collisions. The banging about might therefore go on 

 for a very long time, and the observations made by Dun^r, 

 and classified by Mr. Lockyer, show that this is probably 

 the case. The increase of temperature would accordingly 

 take place gradually, and not suddenly, as Dr. Croll 

 supposes ; and further, the highest temperature would be 

 associated with a certain class of stars, and not with 

 the nebulae themselves. 



The subject of new stars is dismissed with very few 

 words. The general view adopted by Dr. Croll seems to 

 be that in such a case as Nova Cygni the 'outburst was 

 due to the collision of a star with a swarm of meteorites. 

 The spectroscopic evidence in favour of Mr. Lockyer's 

 view, that such an outburst is due to the collision between 

 two swarms of meteorites, is not even referred to. The 

 case of Nova Cygni, indeed, has an important bearing on 

 theories of cosmogony. Its spectrum, as observed by 

 Copeland when it was just fading from our view, was that 

 of a planetary nebula. If, therefore, a nebula is at a 

 higher temperature than a star, Nova Cygni must have 

 got hotter as it got dimmer ! 



Dr. Croll shows that his theory explains other details 

 of the structure of our universe, including the proper 

 motions of stars and the origin of binary systems, but 

 these need not be more than mentioned. 



Assuming that Dr. Croll has established that gravitation 

 alone would have been incompetent to produce the heat 

 originally possessed by the solar nebula, it is only 

 necessary to reconcile this with the low-temperature 

 theory of nebulae, as the high-temperature theory has 

 been shown to be inconsistent with the facts. 



It may be suggested that instead of the dark stellar 

 masses endowed with motion which Dr. Croll supposes to 

 have been the pre-nebular condition, meteorites at great 

 distances apart were endowed with similar velocities. In 

 the first groupings, the collisions would only occur very 

 rarely, and there would be more grazes than anything 

 else, so that the average temperature might still be low 

 in the earlier stages. Prof. G. H. Darwin* has recently 

 shown that the conception of fluid pressure which is 

 demanded by Laplace's nebular hypothesis is not difficult 

 to reconcile with the meteoritic hypothesis. If we substitute 

 meteorites in collision for the molecules of a gas impinging 

 against each other, there would be a quasi-fluid pressure 

 as the average result of the impacts of the meteorites, 

 and the separation of the planets from the meteor-swarm 

 would take place exactly as in a gaseous mass. 



A. Fowler. 



THE TELEPHONE. 

 The Telephone. By William Henry Preece, F.R.S., and 

 Julius Maier, Ph.D. Pp. i -xvi., and 1-498. (London : 

 Whittaker and Co., 1889.) 



1" H I S book is one of the " Specialists' Series " of tech- 

 nical manuals now being issued by Messrs. Whittaker 

 and Co. Its aim is to give as full an account as possible 



' Phil. Trans., vol. cclxxx. pp. 1-69. 



