134 MEMORIE DELLA SOCIETÀ* 



spectnim, the « easiug off • of the absorption being uow oa one side and now oii 

 the other. 



9. Shouid ali these phenomena be nltiniately referred to the causes which produce 

 a chaunellcd-space spectrura (one of which undoubtedly is the tendency to a uni- 

 lateral instead of a bilateral wideuing), a line-spectrum will be regarded as a spe- 

 cial case inerely, and not as an entirely different spectrum, as it has beea hithertoj 

 and the range of molecnlar combinations in any one eiement from which line-spec- 

 tra may be prodiiced is extended. 



10. The questiou fiirther arises, whether, many of the short liues in spectra are 

 not remnants of channelled-space spectra. 



On the Absorption of great TMcknesses of Metallic and Metalloidal 



Yapours 



By J. Norman Lockyer, F.R.S. 



It has been assnmed hitherto that a great thickness of a gas or vaponr causes 

 its radiation, and therefore its absorption, to assume more and more the character 

 of a continuous spectrum as the thickness is increased. 



It has been shown by Dn Frankland and myself that such a condition obtains 

 wheu the density of a vapour is increased, and ray later researches bave shown 

 that it is brought about in two ways. Generalizing the work I have already done, 

 without iutending thereby to imply necessarily that the rule will hold universally, 

 or that it exhausts ali the phenomena, it may be stated that metallic eleraeuts of 

 low specific gravity approach the continuous spectrum by wideuing theirlines, while 

 metallic elemeuts of high specific gravity approach the continuous state by incrcasiug 

 the number of their lines. Hence in the vapours of Na, Ca, Al, and Mg we have a 

 small number of lines which broaden , few short lines being added by increase of 

 deusity; in Fé, Co, Ni, &. we have many lines which do not so greatly broaden, 

 many short lines being added. 



The observations I made in India during the total solar eclipse of 1871 were 

 against the assumption referred to ; and if we are to hold that the lines, both 

 • fundamental » and • short, » which we get in a spectrum , are due to atomic 

 impact (defining by the word atom, provisionally, that mass of matter which gives 

 as a line-spectrum), then, as neither the quantity of the impacts uor the quality 



