ON MATTER, FOECE, AND MOTION IN SPACE. 51 



self-same laws of transmission. We are stopt in limine 

 in this research, and the undulatory theory falls from 

 under us, unless recognising some medium through 

 which these laws may have effect. 1 



The question then arises, ' What is this medium 

 thus universal through space, and how best to be 

 defined ? ' We assume it to be material from the 

 necessity just stated, and the term Ether has been 

 adopted to satisfy, provisionally at least, the need of a 

 name. From the senses we obtain no direct informa- 

 tion as to its existence or properties, and the indirect 

 conclusions of reason are drawn from a limited source. 

 Imponderable we may call it, only in the sense that 

 we cannot weigh it, since, if truly material, we must 

 presume it subject to the common law of gravitation. 

 We may affirm that as a material medium it is of in- 

 finite tenuity the matter composing it being probably 

 simple and elementary in kind, and its atomic parts (for 

 we must needs so name them) of such perfect mobility 

 as to convey the most complex and delicate undulations 

 impressed upon them. Under the same view we may 

 speak of the intense elasticity of the medium we are 

 seeking to comprehend. On this point, and in regard 

 to the existence of a self-repulsive force among the 

 ultimate particles of ether, various hypotheses have 

 been proposed, some of them, as those of Mosotti, 

 aiming at a mathematical form. But no laws have 



1 Aristotle, speaking of the TO /iraw, adds, *} dia TOVTOV KIVTIGIQ iarl 

 ij -jroiovaa TO 6pai>. 



The experiments of Balfour Stewart, indicating a certain heating 

 effect on a disk of aluminium rapidly rotated in an air-pump vacuum, 

 require repetition before being brought in direct proof of a material ether. 



E 2 



