of EdinljUTgli, Session 1883-84. 
701 
long connected system of rigidly similar triplets, extending all across 
the spectrum from Scarlet to Glaucous. Besides which, the whole 
spectrum commences with a notable Ultra Bed line which outflanks, 
or is further towards the invisible red than any line in any other 
gas. 
NITROGEN. 
Nitrogen in vacuum tubes must come in here, for though I may 
have some suspicions that it will be found eventually not to be a 
simple gas, it is practically so to all present science. 
Nitrogen’s tube spectrum then begins only a very little short of 
the Ultra Red line of 0 ; and rapidly rises to remarkable and almost 
continual brilliance in five long and successive groupings of 10 or 11 
bands each. 
This, in a general way, has long been known ; but what I have 
now to add, besides the discovery of the earliest or most Ultra Red 
group, is the resolution of these bands into lines; into hundreds 
and thousands of lines finer, sharper, and set more closely together 
than the lines of any other gas. 
A good idea is given of them in the Plates of Orange and Green 
“ N,” now presented to the Society, and like all the other plates are 
reduced copies of my original measures and drawings, lately executed 
for me by Mr T. Heath, the first Assistant in the Observatory, and 
who has remarkable skill and understanding for such work. 
Of Elemental Gases Generally. 
If an elemented gas, such as that of Carbon, or of Iron, can only 
exist as a permanent gas at the temperature of the condensed electric 
spark, it can pretty evidently have for us only the one gaseous 
spectrum due to that temperature ; for man knows no other higher 
stage. 
But if it be the case of a gas permanent at all temperatures from 
the lowest to the highest, it may have besides the condensed spark 
spectrum, the simple electric spark spectrum, and also what we may 
call the cold spark, or the auroral spectrum. 
Each of these three spectra too, of one element, must, under appro- 
priate circumstances, have its reversal or double ; as from bright 
lines in a dark field to dark lines in a bright field. 
