262 Dr. Collie and Prof. Ramsay. Behaviour of [Feb. 13, 



from that of all other gases. In short, a tube like those termed 

 " PfLiicker's tubes," need not be filled with helium at a low pressure, 

 but shows all the phenomena of a vacuum-tube at ordinary atmo- 

 spheric pressure. The order of arrangement of the other gases, 

 moreover, is not the same as that which obtains with the shower of 

 sparks passing through the gas. This may be due to some molecular 

 change due to lowering of pressure ; but such a supposition is of a 

 purely speculative character. 



Our next series of experiments relate to the visibility of the spec- 

 trum of one gas in presence of another. The gases employed were 

 hydrogen, oxygen, nitrogen (atmospheric), a,rgon, and helium; our 

 primary object was a practical one — to ascertain whether the spec- 

 trum affords an adequate test of the purity of the gases. For 

 example, if the spectrum of nitrogen is no longer visible in a mixture 

 of argon and possibly nitrogen, can it be assumed that nitrogen is 

 wholly absent ? How much must the percentage of nitrogen in such 

 a mixture be reduced before the spectrum disappears? 



To follow how these experiments were carried out, a figure of the 

 apparatus employed is necessary, and a detailed description may 

 prove helpful to others willing to make a similar inquiry. 



The Topler pump A (fig. 2), with connection tube B, and drying 

 tube C (filled with phosphorus pentoxide), had a volume of 290 c.c. 

 up to the stopcock D. The rest of the apparatus, including connect- 

 ing tube E, drying tube F, vacuum tube Gr, and tubes down to stop- 

 cock. H had a volume of 57*5 c.c. The ratio of the total volume 

 to the volume of the vacuum tube, &c, is nearly 6 : 1, and, as extreme 

 accuracy is not here necessary, it will be taken that, on allowing gas 

 to pass the stopcock D and enter the pump, its volume will be 

 increased and its pressure diminished in the proportion of 1 : 6. 



The vacuum tube G was without electrodes ; its wide portions were 

 covered with tinfoil, and connection was made with the coil by loops 

 of wire twisted round the foil. 



The method of introducing the gases requires description. H is a 

 two-way stopcock, and the exit-tubes were curved round and stood in 

 a mercury trough, I. Their ends (one of which is shown at K) were 

 drawn out and bent over as shown in the small figure (3), and a 

 slight scratch with a glass-cutter w T as made at the bend. The little 

 tubes shown in fig. 2 at K were, at a separate mercury trough, 

 filled with the required gases, after being washed out by intro- 

 ducing a few bubbles of the gas in question; they were then trans- 

 ferred to the trough I, and placed in the position shown. The 

 whole apparatus was next pumped free of air ; the nitrogen spec- 

 trum fades, and finally a vacuum is obtained through which a spark 

 will not pass ; the spark prefers to strike across between the coatings 



