xix.] THEORY VERSUS FACT. 269 



of the elementary bodies, the solidarity at once breaks down ; 

 we find that the classification after all is useless, because the 

 same substance may behave as a dyad, a tetrad, a hexad, a 

 pseudo-tryad, and a pseudo-octad ; in fact, one feels one is 

 dealing with something that is more like a moral than a phy- 

 sical attribute a sort of expression of free will on the part of 

 the molecules. We are, I think, justified in asking whether 

 these various attempts to formulate a science do not break 

 down after a certain point, because they attempt to give a 

 fixity to what is in truth variable. 



When we pass to the facts of the science, the key-note of 

 which is variability from one end of the scale to the other, we 

 find that the view of successive dissociation, the view of variable 

 molecular groupings brought about under different conditions, is 

 really more or less in accordance with them, where the laws based 

 on fixity break down entirely. Thus, for instance, let us take the 

 question of vapour densities. The view accounts fully for the 

 so-called anomalous vapour densities, and in this way : it sug- 

 gests that the elements may really be complex groups which 

 break up into their constituent groups under suitable conditions 

 of temperature, as phosphoric chloride and many other bodies 

 do when obtained in the condition of vapour. We have dis- 

 similar groups in the one case, and possibly similar groups in 

 the other. In this way, that contradiction in terms, the " mon- 

 atomic molecule," really becomes the evidence of a higher law. 



Let us pass to allotropic conditions. The explanation of 

 these is that there are bodies which have a large molecular 

 range within the ordinary temperatures at our command. The 

 substances in which allotropism is most marked are all metal- 

 loids which have not been found in the sun, and the allotropic 

 forms give us in many cases different spectra spectra indicating 

 a considerable complexity of the molecules which produce them 

 about which something has been said already. In the passage 

 from one allotropic condition to the other, energy, without any 



