xxi.] MORE EXPERIMENTS 295 



seen alone in a vacuum tube, is enough for my present 

 purpose. 



With regard to the second point ; the passage from the heat- 

 level of the flame to that of the spark after volatilisation is 

 complete produces no visible effect, indicating that in all proba- 

 bility the effects heretofore ascribed to quantity have been due 

 to the presence of the molecular groupings of greater complex- 

 ity. The more there is to dissociate, the more time is required to 

 run through the series, and the better the first stages are seen. 



Experiments lased on the Second Consideration. 



The second experimental method deals with vapours of ele- 

 mentary bodies volatilised at different temperatures in vacuum 

 tubes. Many of the phenomena are quite new, and lines thus 

 seen alone and of surpassing brilliancy, are those seen as short 

 and faint in ordinary methods of observation. 



The novelty of the method consists in the use of the luminous 

 electric current as an explorer and not as an agent for the supply 

 of the vapours under examination ; that is to say, the vapours 

 are first produced by an external source of heat, and are then 

 rendered luminous by the passage of the current. The length 

 and bore of the tube therefore control the phenomena to a 

 certain extent. 



A form of apparatus which I have found to answer very well 

 is shown in the accompanying woodcut (Fig. 99). 



A i s the tube or retort containing the metal experimented on 

 in its lower extremity, and having a platinum wire sealed into it 

 at a distance of about two inches from the lower end, the other 

 e nd being drawn out and connected by a mercury joint to an 

 ordinary Geissler tube, which is connected by another mercury 

 joint to the Sprengel pump c. 



Another form of tube which I have used is prepared by in- 

 serting two platinum poles into a piece of combustion tubing 



