4.V2 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



shape may appear, viz., the temperature of the condensed spark, the simple 

 spark and the atmospheric or auroral. 



With roii) pound gases, there are only the two latter temperature stages, viz., 

 the simple spark and the cold auroral; for the high-temperature condensed 

 spark resolves them instantly into their known chemical components, which 

 then give out their own elemental spectra. While with Carbon, and every other 

 similar solid, there is only one temperature stage; viz., that highest one at 

 which alone it can be volatilized. 



To return then to the elemental and permanent gases, as the completer 

 system, how little do we know yet of all three varieties of spectra belonging 

 to any one of them; — not to say anything of each variety, in order to be fully 

 understood, requiring to be made to appear first as an emission spectrum with 

 bright lines in a dark field, and second, as an absorption spectrum, with the 

 same set of lines but dark in a bright field. 



Suppose we take Oxygen again as an example. 



1. Its emission spectrum of bright lines in the condensed spark, or jar 

 discharge has been grandly studied by Kirchoff, Thalen, Plucker, and 

 Huggins in long past years, with a most satisfactory cataloguing of Wave- 

 length places again and again, — and yet it was left for me to discover the 

 earliest of its ultra red lines last summer. But no one has yet seen either that 

 line, or I suppose most of the others, as dark, or absorption, lines; though 

 Professors Liveing and Dewar are now working at that subject, and towards 

 that end very magnificently in the Cavendish Laboratory at Cambridge. 



2. Oxygen's emission spectrum in the simple spark, viz., the spectrum of 

 minute triplets and a few thin lines, has been set forth in this paper at some 

 length, though elsewhere, and particularly in London, only 4 lines of it have 

 been recognised ; but none of them have yet been seen by any one as a dark 

 absorj)tion spectrum, so far as I am aware. * 



3. At the atmospheric, the cold, or auroral, temperature no one has ever 

 yet seen any bright, or emission, spectrum of Oxygen. But two persons are 

 said to have recently seen its dark, or absorption lines connected with that very 

 low, or non-fiery, temperature; and it came about in this manner. 



After I had for years and years besought, but in vain, the rich London 

 Societies, or the Government to make the enormous experiments which are 

 necessary for the purpose, — these have recently been made in St Petersburg ! 

 There, in connection with the University of that city, M. Egoroff, with his 

 friend M. Khamantoff, — so far as we can trust the rather too scanty informa- 

 tion yet given out, — established a horizontal tube 66 feet long with glazed 



* I thought, on the first discovery of 3 of these triplets, that they could be recognised in 

 Angstrom's Normal Solar Map as dark Fraunhofer lines, but I delay now either affirming or refuting 

 that idea, until I have made more satisfactory and exact observations on the Solar Spectrum itself. 



