360 



NA TURE 



[Mar. 12, 1874 



" How can a volcanic theory of the lunar phenomena be 

 upheld consistently with the condition that it possesses 

 no atmosphere to support Fire ? " 



In the chapter on the cooling of the crust (Chap. III.), 

 special attention is directed to observations tending to 

 show that cast-iron and even slag and Vesuvian lava 

 expand on cooling. This will be new to physicists : — 



"The broad general principle of the phenomenon 

 here referred to is this : That fusible substances are (with 

 few exceptions) specifically heavier while in their molten 

 condition than in the solidified state, or in other words, 

 that molten matter occupies less space, weight for weight, 

 than the same matter after it has passed from the melted 

 to the solid condition. It follows as an obvious corollary 

 that such substances contract in bulk in fusing or melting, 

 and expand in becoming solid. It is this expansion upon 

 solidification that now concerns us. 



"Water, as is well known, increases in density as it cools, 

 till it reaches the temperature of 39° F., after which, upon 

 a further decrease of temperature, its density begins to 

 decrease, or in other words, its bulk expands, and hence 

 the well-known fact of ice floating in water, and the in- 

 convenient fact of water-pipes bursting in a frost. This 

 action in water is of the utmost importance in the grand 

 economy of nature,and it has been accepted as a marvel- 

 lous exception to the general law of substances increasing 

 in density (or shrinking) as they decrease in ternperature. 

 Water is, however, by no means the exceptional sub- 



stance that it has been so generally considered. It is a 

 fact perfectly familiar to iron-founders, that when a mass 

 of solid cast-iron is dropped into a pot of molten iron of 

 identical quality, the solid is found to float persistently 

 upon the molten metals — so persistently that when it is 

 intentionally thrust to the bottom of the pot, it rises again 

 the moment the submerging agency is withdrawn " (p. 20). 



There will be many for whom this part of the work will 

 possess great interest, but I take it few will accept the 

 startling conclusions drawn from the assert.;d expansion. 



" This expansion of volume which accompanies the 

 solidification of molten matter furnishes a key to the solu- 

 tion of the enigma of volcanic action ; and that such 

 theories as depend upon the agency of gases, vapours, or 

 water are at all events untenable with regard to the moon, 

 where no gases, vapour, or water appear to exist " (p. 27). 



I will return to this point presently, but meantime let 

 us follow the contracting globe. Messrs. Nasmyth and 

 Carpenter quite accept tangential pressure as being the 

 only true cause of elevation, and its effect is very well put : 



" When the molten sub-stratum had burst its confines, 

 ejected its superfluous matter, and produced the resulting 

 volcanic features, it would, after final solidification, resume 

 the normal process of contraction upon cooling, and so 

 retreat or shrink away from the external shell. Let us now 

 consider what would be the result of this. Evidently the 

 external shell or crust would become relatively too large 



to remain at all points in close contact with the subjacent 

 matter. The consequence of too large a solid shell having 

 to accommodate itself to a shrunken body underneath, is 

 that the skin, so to term the outer stratum of solid matter, 

 becomes shrivelled up into alternate ridges and depres- 

 sions or wrinkles" (p. 28). 



The preceding extracts will give an idea of the authors' 

 view of the general phenomena which accompanied the 

 cooling of the crust. We have no atmosphere, the crust 

 as it cools expands and cracks, and through these cracks 

 the interior liquid is ejected, and finally tangential pres- 

 sure does the rest. 



Now leaving the question of the atmosphere for a time, 

 let us compare this with Mr. Mallet's reasoning. As 

 a planet cools, the crust thickens, and the nucleus con- 

 tracts ; the crust must follow the nucleus, hence tangential 

 pressure and its concomitants, elevation where possible, 

 where not possible then tremendous interior motions, 

 which motions are converted into heat, which heat + water 

 produces volcanic activity. Further, the smaller a globe 

 is, the more rapidly will it cool, and the more marked will 

 the phenomena which accompany coohng be. Hence Mr. 

 Mallet's hypothesis is competent to explain all the extreme 

 development of volcanic activity on the moon by exactly 

 similar causes which we know to have gone on here. 



Now as I have said, Mr. Mallet wants water for his 

 volcanoes, both here and on the moon, but Messrs. 

 Nasmyth and Carpenter will not even allow that an atmo- 

 sphere, still less water, has ever existed there. Nowhere 

 1 unhesitatingly range myself on the side of Mr. Mallet. 

 1 believe in an absolute uniformity throughout all Nature 

 in such matters. I do not mean uniformity of matter, so 

 far as chemical materials go, but of manner. 



Now what is an atmosphere.' or to put the question 

 more specifically, what is our atmosphere ? Is it not a 

 residue .' We have free oxygen in the atmosphere at the 

 present time ; had we not very much more before the 

 various metals which now exist in combination with that me- 

 talloid existed in their pure state.' Now how has combina- 

 tion been brought about ? By exposing the metals to the 

 atmosphere and its contained oxygen. Now suppose the 

 machinery, the function of which in past time has been to 

 bring these metals to the surface, had been a thousand 

 times more powerful, would there be as much oxygen in 

 the air now as there is .' A child can answer this question, 

 and it is one of several which might be asked all tending 

 to show that it is as unnecessary as it is unphilosophical 

 to supposethat there never was a lunar atmosphere, because 

 there is only a tenuous one at the best now. I shall not 



