2 
DES. J. PLUCKEE AND J. W. HITTOEF ON THE 
starting off from them, are volatilized: even in the gaseous state they conduct the 
electric current from point to point, and exhibit, while heated by it, the characteristic 
spectral lines of the metal. In all experiments made in this way, either air or another 
permanent gas occupied the space between the two extremities of the wires. The con- 
sequence of this is, the interposed gas partly conducting the electric current on its way 
through it, two spectra are obtained at the same time — the spectrum of the metal and 
the spectrum of the interposed gaseous medium. This inconvenience is the greater, as 
in most cases the number of bright lines constituting gas-spectra is a considerable one ; 
it is least in the case of hydrogen, the spectrum of which, if appearing under these con- 
ditions, becomes nearly a continuous one (59). If the substance submitted to experi- 
ment be not a metal or charcoal, the extremities of the metallic wires are to be covered 
with it. Then we get with the spectrum of the non-conducting substance at the same 
time the spectrum of the metal covered by it. 
4. The spectra are obtained the most beautifully and are the most suitable for exami- 
nation in their minute details, if the substance be in the gaseous state before the electric 
discharge is sent through it. The spectral tubes for enclosing gas, first proposed and 
employed by one of us, were in most cases, with some modifications, adopted for our more 
recent researches. Our tubes, as represented by the diagram (fig. 1), gene- ^ 
rally consist of a capillary middle part 30-40 millims. long, and T5-2 millims. 
in diameter, forming a narrow channel, by which two larger spheres, with 
platinum electrodes traversing the -glass, communicate with one another. 
The small tube starting from one of the spheres serves to establish the com- 
munication with the exhauster, to which it is either attached by means of a 
cement (sealing-wax for instance), or soldered by the blowpipe. The ex- 
hauster, made solely of glass, without any metal, is connected with an addi- 
tional system of glass tubes and glass cocks, by means of which the spectral 
tube is most easily filled with the gas to be examined. If the gas be a per- 
manent one, the apparatus by which it is developed, and its accessory parts, 
by which it is purified and dried, may, as well as the spectral tube, simulta- 
neously and separately be evacuated. The gas arrives directly from the appa- 
ratus into the tube, which, ad libitum , may be alternately filled and ex- 
hausted again. Finally, the tension of the gas is regulated and measured 
by means of a manometer in connexion with the exhauster. 
5. In order to compare with one another the spectra corresponding to different 
densities of the gas, or even to a mixture of different gases, the tube may be examined 
by the spectroscope while attached to the exhauster. But generally the spectral tube 
was blown off and hermetically sealed at the extremity of the narrow tube starting 
from one of the spheres. This tube equally serves to attach the spectral tube before 
the slit of the spectroscope. 
6. If the substance submitted to examination were at the ordinary temperature in 
the liquid or solid condition, the tube destined to receive it was made of a glass diffi- 
