1874.] Mr. J. N. Lockyer's Spectroscopic Notes. No. I. 371 



V. " Spectroscopic Notes. No. I. On the Absorption of great 

 Thicknesses of Metallic and Metalloidal Vapours." By J. 

 NORMAN LOCKYER, JF.R.S. Received April 20, 1874. 



It has been assumed hitherto that a great thickness of a gas or vapour 

 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 Dr. Frankland and myself that such a condition 

 obtains when the density of a vapour is increased, and my later researches 

 have shown that it is brought about in two w r ays. Generalizing the 

 work I have already done, without intending thereby to imply necessarily 

 that the rule will hold universally, or that it exhausts all the phenomena, 

 it may be stated that metallic elements of low specific gravity approach 

 the continuous spectrum by widening their lines, while metallic elements of 

 high specific gravity approach the continuous state by increasing 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 density ; in Fe, Co, Ni, &c. 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 us a line-spectrum), then, as neither the quantity 

 of the impacts nor the quality is necessarily altered by increasing the 

 thickness of the stratum, the assumption seems also devoid of true 

 theoretical foundation. 



One thing is clear, that if the assumed continuous spectrum is ever 

 reached by increased thickness, as by increased density, it must be reached 

 through the " short-line " stage. 



To test this point I have made the following experiments : 



1. An iron tube about 5 feet long was filled with dry hydrogen ; 

 pieces of sodium were carefully placed at intervals along the whole 

 length of the tube, except close to the ends. The ends were closed with 

 glass plates. The tube was placed in two gas-furnaces in line and 

 heated. An electric lamp was placed at one end of the tube and a 

 spectroscope at the other. 



When the tube was red-hot and filled with sodium-vapour throughout, 

 as nearly as possible, its whole length, a stream of hydrogen slowly 

 passing through the tube, the line D was seen to be absorbed; it was no 

 thicker than when seen under similar conditions in a test-tube, and far 

 thinner than the line absorbed by sodium- vapour in a test-tube, if the 

 density be only slightly increased. 



