IS 



CHAP. II. SOME PIONEERING DIFFICULTIES. 



I began to endeavour to apply the principles of spectrum 

 analysis to the investigation of the nature of the heavenly bodies in 

 1865, the then idea, based upon Kirchhoff and Bunsen's work of 

 1859, was that the spectrum of a chemical element was one and in- 

 divisible that it could not be changed by temperature or by anything 

 else. 



Looking back it is easy to see now that this idea largely depended 

 upon the fact that in the early days low j flame temperatures were 

 generally employed, and that it so happens that the substances best 

 visible in the flame and which were therefore chosen to experiment 

 upon, such as sodium, calcium, potassium and the like, give us line 

 spectra at low stages of heat. 



Hence the first spectroscopic ideas entirely agreed with those of 

 the chemist, that the chemical " atom," defined by a certain " atomic "" 

 weight was a manufactured article, indivisible, indestructible. Chemi- 

 cal elementary substances were either composed of these atoms, these 

 indivisible units ; or of " molecules " consisting of two or more of 

 them, hence the terms " diatomic " and " polyatomic " molecule. 



The difference between the spectra of the same element in the solid 

 and gaseous, states, in which we have first a continuous and secondly a line 

 spectrum, was ascribed to the restricted motion of the atom in the solid 

 and its freedom in the gaseous state it was a question of " free path." 

 The difference between the states which gave us the continuous and dis- 

 continuous spectra was a physical difference having nothing to do with 

 chemistry. According to the kinetic theory of gases, the particles of all 

 bodies are in a state of continual agitation, and the difference between 

 the solid, liquid, and gaseous states of matter is that in a solid body the 

 molecule never gets beyond a certain distance from its initial position. 

 The path it describes is often within a very small region of space. Prof. 

 Clifford, in a lecture upon atoms, many years ago illustrated this very 

 clearly. He supposed a body in the middle of a room held by elastic 

 bands to the ceiling and the floor, and in the same manner to each side 

 of the room. Now pull the body from its place ; it will vibrate, but 

 always about a mean position ; it will not travel bodily out of its 

 place ; it will always go back again. 



We next come to liquids. Concerning these we read : "In fluids, 

 on the other hand, there is no such restriction to the excursions of a 



