INTRODUCTION. XXI 



a solid body than a gas, resembling a gelatinous mass rather than the 

 air. It participates in the luminous phenomena due to the vibrations 

 of the atoms of the fixed stars, and hence it is the transmitter of 

 light, which is conducted by means of its vibrations, with inconceivable 

 rapidity (42,220 geographical miles per sec.) to our visual organs 

 (Tyndall). 



Imponderable matter (ether) and ponderable matter are not 

 separated sharply from each other; rather does the ether penetrate 

 into all the spaces existing between the smallest particles of ponderable 

 matter. 



Particles. Supposing that ponderable matter were to be sub- 

 divided continuously into smaller and smaller portions, until we 

 reached the last stage of division in which it is possible to recognise 

 the aggregate condition of the matter operated upon, we should call 

 the finely-divided portions of matter in this state particles. Particles 

 of iron would still be recognised as solid, particles of water as fluidj 

 particles of oxygen as gaseous. 



Molecules. Supposing, however, the process of division of the 

 particles to be carried further still, we should at last reach a limit 

 beyond which, neither by mechanical nor by physical means, could 

 any further division be effected. We should have arrived at the 

 molecules. A molecule, therefore, is the smallest amount of matter 

 which can still exist in a free condition, and which as a unit no 

 longer exhibits the aggregate condition. 



Atoms. But even molecules are not the final units of matter, since 

 every molecule consists of a group of smaller units, called atoms. An 

 atom cannot exist by itself in a free condition, but the atoms unite 

 with other similar or dissimilar atoms to form groups, which are 

 called molecules. Atoms are incapable of further sub-division, hence 

 their name. We assume that the atoms are invariably of the same 

 size, and that they are solid. From a chemical point of view, the 

 atom of an elementary body (element) is the smallest amount of 

 the element which can enter into a chemical combination. Just as 

 ponderable matter consists in its ultimate parts of ponderable atoms, 

 so does the ether consist of analogous small ether-atoms. 



Ponderable and Imponderable Atoms. The ponderable atoms within 

 ponderable matter are arranged in a definite relation to the ether- 

 atoms. The ponderable atoms mutually attract each other, and 

 similarly, they attract the imponderable ether-atoms; but the ether- 

 atoms repel each other. Hence, in ponderable masses, ether-atoms 

 surround every ponderable atom. These masses, in virtue of the 

 attraction of ,the ponderable atoms, tend to come together, but only 



