494 Dr. Arthur Schuster [Jan. 28, 



surpasses our imagination, so is tliere no other instrument whicli equals 

 it in the information it can yield on the minute particles at the other 

 end of the scale, particles which in their turn are so small, that we can 

 form no conception of their size or number. The range of the 

 spectroscope includes both stars and atoms, and it is about these 

 latter that I wish to speak. 



The idea that all matter is built up of atoms, which we cannot 

 further divide by physical or chemical means, is an old one. As a 

 scientific hypothesis, however — that is, an hypothesis which shall not 

 only qualitatively, but also quantitatively, account for actual phenomena 

 — it has only been worked out in the last thirty years. The develop- 

 ment of molecular physics was contemporaneous with that of spectro- 

 scopy, but the two sciences grew up independently. Those who 

 strove to advance the one paid little attention to the other, and did 

 not trouble to know which of their conclusions were in harmony, 

 which in discordance, with the results of the sister science. It is 

 time, I think, now that the bearing of one branch of inquiry on the 

 other should be pointed out : where they are in agreement, their con- 

 clusions will be strengthened, while new investigations will lead to 

 more perfect truths where disagreement throws doubt on apparently 

 well-established principles. 



What I have ventured to call modern spectroscopy, is the union of 

 the old science with the modern ideas of the dynamical theory of 

 gases, and includes the application of the spectroscope to the experi- 

 mental investigation of molecular phenomena, which without it might 

 for ever remain matters of speculation or of calculation. 



A body, then, is made up of a number of atoms. These are 

 hardly ever, perhaps never, found in isolation. Two or more of 

 them are bound together, and do not part company as long as the 

 physical state of the body remains the same. Such an association of 

 atoms is called a molecule. When a body is in the state of a gas or 

 vapour, each molecule for the greater part of the time is unaffected 

 by the other molecules in its neighbourhood, and therefore behaves as 

 if these were not present. The gaseous state, then, is the one in which 

 we can best study these molecules. They move about amongst each 

 other, and within each molecule the atoms are in motion. Each 

 atom, again, has its own internal movement. But if the world was 

 made up of atoms and molecules alone, we should never know 

 of their existence ; and to explain the phenomena of the universe, we 

 must recognise the presence of a continuous universal medium pene- 

 trating all space and all bodies. This medium, which we call the 

 luminiferous ether or simply the ether, serves to keep up the connec- 

 tion between atoms or molecules. All communications from one atom 

 to another and from one molecule to another are made through this 

 other. The internal motions of one atom are communicated to this 

 medium, propagated through space, until they reach another atom ; 

 attraction, repulsion, or some other manifestation takes place ; and if 

 you examine any of the changes which you see constantly going on 



