6 
DRS. J. PLUCKER AND J. W. HITTORF ON THE 
the breadth of the slit as observed without the interposed prisms, are either superposed, 
or touch one another, or are separated by a black space. In making use of the two 
prisms, we generally regulated the aperture of the slit so that the two small sodium- 
bands appeared separated by a black space having nearly the breadth of these bands. 
In this case the angle at which the aperture of the slit is seen is equal to half the angu- 
lar distance of the two middle lines of the bands, and therefore equal to half the angu- 
lar distance of the two sodium-bands themselves after being reduced by narrowing the 
slit to mathematical lines. 
If the images touch each other, the aperture of the slit and the two sodium-lines are 
seen at the same angle. 
14. The first fact which we discovered in operating with our tubes, guided by the 
above explained principles, was the following one : — 
There is a certain number of elementary substances , which , when differently heated , fur- 
nish two kinds of spectra of quite a different character , not having any line or any band 
in common. 
The fact is important, as well with regard to theoretical conceptions as to practical 
applications — the more so as the passage from one kind of spectra to the other is by no 
means a continuous one, but takes place abruptly. By regulating the temperature you 
may repeat the two spectra in any succession ad libitum. 
We will now treat more explicitly the case of Nitrogen , which first unfolded to us its 
different spectra. These spectra, obtained in the easiest and most striking way, have 
been examined by us in every point of view. The other cases of double spectra may 
hereafter be spoken of in a more summary manner. 
15. We examined nitrogen prepared in different ways, even in the state of greatest 
purity ; but we found that, in order to get pure spectra of it, it was not necessary to 
free the gas from all traces of air *. Therefore we may select the following prepara- 
tion, imperfect as it is, in order to give an instance of constructing nitrogen-tubes. 
Three absorbing apparatus were connected with one another and, by means of a stop- 
cock, with the exhauster, the first two being filled with a solution of pyrogallic acid 
in hydrate of potash, and the third with concentrated sulphuric acid. After having 
evacuated the interior of the exhauster and the spectral tube connected with it, by care- 
fully turning the stopcock air was very slowly admitted, leaving its oxygen and carbonic 
acid to the first two, and its aqueous vapour to the third absorbing apparatus. Thus by 
and by the exhauster, with the tube, was filled with nitrogen, the manometer always 
indicating the tension of the gas. These operations being repeated several times by 
alternately evacuating and introducing new nitrogen, finally, the tension of the gas 
* Whatever may he, under certain conditions, the practical importance of prismatic analysis in detecting 
certain substances converted into vapour, whatever may be its use in indicating traces of a single gas imper- 
ceptible by other means, mixtures of permanent gases are not fitted to be examined by the prism. A gas, if 
mixed in rather small proportion with another one, entirely escapes observation. The proportion necessary to 
render it visible depends upon the nature of the gas as well as upon the temperature of ignition. 
