ON OUR KNOWLEDGK OF SPECTUUM ANALYSIS. 281 



-widen the lines by an increase in the intensity of the discharge.' Those 

 who believe that the widening of a line is due to an increase of pressure, 

 attribute to an increase in temperature, such as is produced by an in- 

 creased discharge, an influence only in so far as it raises the pressure of 

 the gas. This opinion is supported by the fact that the sodium-lines 

 widen rather more easily at low temperatures. 



According to the molecular theory of gases, the following explanation 

 might be given for the widening of lines. 



As long as a molecule vibrates by itself, uninfluenced by any other 

 molecule, its vibrations will take place in regular periods. The lines of 

 its spectrum will consequently be sharp. But if the molecule is placed 

 in proximity with others, its vibrations will be disturbed by occasional 

 encounters. The number and strength of these collisions will depend on 

 the pressure of the gas. Ideas analogous to these seem to have been in 

 the minds of many writers, and it is difficult to decide where they first 

 occurred ; but we may quote a short passage taken out of a paper by 

 Lippich,^ in which similar views were, perhaps for the first time, clearly 

 expressed, and in which the reference to Boyle's law is especially in- 

 teresting : — 



' If the pressure of a gas is increased, or if, as in the case of vapours, 

 its properties are not those any more of a perfect gas ; that is, if the 

 length of the path during which a molecule is within the influence 

 of another is not any more small compared to the length of the mean 

 free path, changes in the spectrum will necessai-ily accompany the new 

 state of things. . . . New vibrations will arise, the intensity of which 

 will be the smaller, the further removed they are from the vibrations of 

 the molecule in the ideal state. The lines of the spectrum will then 

 appear with indistinct edges and expand the more, the more the gas 

 deviates from the laws of Mariotte and Gay-Lussac' 



The behaviour of the two yellow sodium-lines is in many respects 

 remarkable. It has already been mentioned that the widening seems to 

 take place more easily at a lower temperature, but it is obviously not 

 due to a lowering of temperature, for Profs. Liveing and Dewar' have 

 observed that a layer of sodium vapour about 4 cm. thick, at atmo- 

 spheric pi-essure, gave sharp and narrow lines, at a temperature which was 

 lower than that of a Bunsen burner; while in the Bunsen a much smaller 

 quantitj' of sodium vapour will produce winged lines. Some of the ex- 

 periments described in the paper to which we have just referred are not 

 easily reconciled with the explanation given above for the widening of 

 lines. Profs. Liveing and Dewar describe the efl'ects of pressure thus : — 



' The efl'ects of compressing the vapour were very remarkable. As 

 the pressure increased the channelled spectrum speedily disappeared, then 

 the diffused edges of the D band contracted, the band itself likewise 

 contracting until it became a very fine pair of lines, or if the amount of 

 sodium present was not too much, D came out bright. On letting ofi" 

 the pressure, the phenomena recurred in the reverse order, and the whole 

 could be repeated several times. After compression, as long as the 

 pressure was sustained, the D absorption remained permanently narrowed, 

 but did not continue bright.' 



' Pogg. Ann. cxxxix. p. 465 (1870). 



* The widening observed by Stearn & Lee {Proc. Soy. Sac. xxi. p. 282 — 1873) is 

 due to this cause. 



' P?'oc. Poy. Sue. xxix. p. 482. 



