218 THE POPULAR SCIENCE MONTHLY. 



But let us examine the consequences of the really interesting t .^po- 

 sition I have just quoted. I presume that it is a law of Nature that " a 

 straight line is the shortest distance between two points." This law 

 affirms the constant association of a certain fact of form with a certain 

 fact of dimension. Whether the notion of necessity which attaches 

 to it has an a 2^'iori or an a posteriori origin is a question not relevant 

 to the present discussion. But I would beg to be informed, if it is 

 necessary, where is the " compelling force " out of which the necessity 

 arises ; and, further, if it is not necessary, whether it loses the charac- 

 ter of a law of Nature ? 



I take it to be a law of Nature, based on unexceptionable evidence, 

 that the mass of matter remains unchanged, whatever chemical or other 

 modifications it may undergo. This law is one of the foundations of 

 chemistry. But it is by no means necessary. It is quite possible to 

 imagine that the mass of matter should vary according to circum- 

 stances, as we know its weight does. Moreover, the determination of 

 the " force " which makes mass constant (if there is any intelligibility 

 in that form of words) would not, so far as I can see, confer any more 

 validity on the law than it has now. 



There is a law of Nature, so well vouched by experience that all 

 mankind, from pure logicians in search of examples to parish sextons 

 in search of fees, confide in it. This is the law that " all men are 

 mortal." It is simply a statement of the observed order of facts that 

 all men sooner or later die. I am not acquainted with any law of 

 Nature which is more " constant and uniform" than this. But will 

 any one tell me that death is "necessary"? Certainly there is no a 

 })riori necessity in the case, for various men have been imagined to be 

 immortal. And I should be glad to be informed of any " necessity " 

 that can be deduced from biological considerations. It is quite con- 

 ceivable, as has recently been pointed out, that some of the lowest 

 forms of life may be immortal, after a fashion. However this may be, 

 I would further ask, supposing " all men are mortal" to be a real law 

 of Nature, where and what is that to which, Avith any propriety, the 

 title of " compelling force " of the law can be given ? 



On page GO, the Duke of Argyll asserts that the law of gravitation 

 " is a law in the sense not merely of a rule but of a cause." But this 

 revival of the teaching of the "Vestiges" has already been examined 

 and disposed of ; and, when the Duke of Argyll states that the " ob- 

 served order," which Kepler had discovered, was simply a necessary 

 consequence of the force of "gravitation," I need not recapitulate the 

 evidence which proves such a statement to be wholly fallacious. But 

 it may be useful to say once more that, at this present moment, nobody 

 knows anything about the existence of a force of gravitation apart 

 from the fact ; that Newton declared the ordinary notion of such force 

 to be inconceivable ; that various attempts have been made to account 

 for the order of facts we call gravitation, without recourse to the no- 



