THE TEACHINGS OF MODERN SPECTROSCOPY. 469 



but also the possibly changed properties, under these conditions, of 

 our terrestrial elements. The spectroscope is rapidly becoming our 

 thermometer and pressure-gauge ; it has become a physical instru- 

 ment. 



The application of the spectroscope to the investigation of the 

 nature of celestial bodies has always had a great fascination to the 

 scientific man as well as to the amateur ; for in stars and nebulse one 

 may hope to read the past and future of our own solar system. But it 

 is not of this application that I wish to speak to-day. 



As there is no other instrument which can touch the conditions of 

 the most distant bodies of our universe, bodies so large that their size 

 surpasses our imagination, so is there no other instrument which 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 spec- 

 troscope 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 can not 

 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 phe- 

 nomena it has only been worked out in the last thirty years. The 

 development of molecular physics was contemporaneous with that of 

 spectroscopy, 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 conclusions 

 will be strengthened, while new investigations will lead to more per- 

 fect truths where disagreement throws doubt on apparently well-estab- 

 lished 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 vapor, each mole- 

 cule for the greater part of the time is unaffected by the other mole- 

 cules in its neighborhood, 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 among each other, and 



