214 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
In the preceding pages it has been shown that the number of strata varies with 
the pressure of the gas, beginning with one luminosity, then increasing in number 
up to a certain point as the pressure is diminished ; as the gas becomes gradually 
attenuated the strata become thicker and fewer ; lastly, the current passes with 
increasing difficulty, and the strata have a tendency to run together. With the 
mercurial pump alone we have not been able to obtain a lower pressure than 2’6 M, 
but with the employment of spongy palladium in a hydrogen vacuum we have reduced 
the pressure to 0'000055 m.m., 0’066 M. 
Tube 145, Hydrogen. 
Length 25 inches, 20 inches between the terminals, one a wire, the other a ring, 
diameter If inch. 
This tube is represented at 145, fig. 3 7. It has an absorption chamber containing 
spongy palladium, obtained by heating its cyanide to redness ; this chamber is 
inches long, f inch in diameter, and is about half-full of palladium, which is held in 
its place by plugs of palladium-foil. At one end of the tube is a short capillary 
calibrated tube contained between two glass stop -cocks, the first communicating with 
the tube, the second with the hydrogen generator. 
The tube was repeatedly exhausted and filled with dry hydrogen, while the 
absorption chamber was being heated in a copper air-bath to a temperature above the 
range of the mercurial thermometer ; a double saddle-screen of tinned iron, placed on 
the small tube connecting the absorption chamber with the glass cock, effectually 
protected the latter from excessive heat. At various pressures from 35 m.m., 
46,053 M,* at which the current of 11,000 cells just passed where the ring was 
positive, most of the phenomena recorded for tube 129 were obtained, but the following 
are the experiments for which the tube was specially constructed. 
In the first instance, in order to test the absorbing power of the palladium, it was 
heated for some hours, and the tube was very completely exhausted ; it was then 
filled with hydrogen at the atmospheric pressure, and kept at this pressure while the 
palladium cooled. The tube was exhausted again, and it was found that at 10 m.m. 
the palladium gave off hydrogen sensibly at ordinary temperatures ; then the air-bath 
was heated above the range of the mercurial thermometer, when the gas filled the 
* Moreen, A. (‘ Comptes Rendus,’ liv., 1862, p. 736) states that he obtained a current in hydrogen at a 
pressure of 24 m.m., which caused a deviation of 1° of his galvanometer ; at a pressure of 2‘8 m.m. the 
deviation was at a maximum 46°, and at 0'06 m.m. the current was less, the deviation being 30°. He 
gives the currents also in carbonic anhydride, nitrogen, and carbonic oxide. In the Ann. de Chim., iv., 
1865, pp. 320-352, is an elaborate investigation on the electric conductibility of gases at very low 
pressures, by this author, the pressure of the gas being estimated by allowing a known volume of gas to 
enter a tube of known capacity in which had been produced a torricellian vacuum ; he found (p. 333) 
the transmission of electricity to cease at a pressure of 0'0037 m.m. with nitrogen. 
