April 6, 1899] 



NA TURE 



535 



A CHAPTER IN THE HISTORY OF 

 SPECTRUM ANALYSIS. 



WHEN 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 



Fig. t. — Spectroscope attached to a large refractor which throws an image 

 of the sun on the slit plate. 



that the spectrum of a chemical element was one and 

 indivisible — 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 

 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 manu- 

 factured article, indivisible, in- 

 destructible. Chemical ele- 

 mentary substances were either 

 composed of these atoms, these 

 indivisible units, or of "mole- 

 cules " consisting of one or two 

 of them, hence the terms "mona- 

 tomic ■' and "diatomic" mole- 

 cule. 



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 discontinuous 



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 differenfe 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, m a lecture 

 upon atoms, manyyears.agb illustrated this very clearly. 

 He supposed a body in the middle of a room held by 

 elastic bands tothe 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' traVet-bWily out of its place ; it 

 will always go backagairt. '' ' ' ■ 



We next come to fluids. Concerning these we read : 

 " In fluids, on the other hand, there is no such restriction 

 to the excursions of a molecule. It is true that the 

 molecule generally can travel but a very small distance 

 before its path is disturbed by an encounter with some 

 other molecule ; but after this encounter, there is nothing 

 which determines the molecule rather to return towards 

 the place from whence it came than to push its way into 

 new regions. Hence in fluids the path of a molecule is 

 not confined within a limited region, as in the case of 

 solids, but may penetrate to any part of the space 

 occupied by the fluid. 



Now we have the motion of the molecule in the solid 

 and the fluid. How about the movement in a gas ? "A 

 gaseous body is supposed to consist of a large number 

 of molecules moving very rapidly." For instance, the 

 molecules of air travel about twenty miles in a minute. 

 " During the greater part of their course these molecules 

 aie not acted upon by any sensible force, and therefore 

 move in straight lines with uniform velocity. When two 

 molecules come within a certain distance of each other, 

 a mutual action takes place between them which may be 

 compared to the collision of two billiard balls. Each 

 molecule has its course changed, and starts in a new path. 



The collision between two molecules is defined as an 

 "encounter"; the course of a molecule between en 

 counters a "free path." " In ordinary gases the free 

 motion of a molecule takes up much more time than is 

 occupied by an encounter. As the density of the gas 

 increases the free path diminishes." 



..—The first method of work with the slit of the spactroicop: clos 

 ent illustrated the light source is an electric .spark produced by 

 ircuit. The slit end of the collimator is shown to the right. 



to the light source. In the experi- 



in induction coil with Leyden j.ir in 



It will b2 sejn at once that on the view first held that the 

 difference between continuous and discontinuous spectra 

 depended simply upon the solid and gaseous states, no 

 solid could give us a line spectrum ; and the well-known 

 absorption spectra of didymium glass and other solid 

 bodies would be impossible. 



NO. 1536, VOL. 59] 



