226 



Royal Society : — 



worth further investigation. The result of the experiments I have 

 made with this object forms the subject of the present communication. 

 5. After verifying the fact that the heating of the negative terminal 

 of an induction coil is always obtained either in a carbonic acid va- 

 cuum, in rarefied gases, or in air, provided the terminal wire is thin, 

 but not if thick, I proceeded to examine with greater precision than 

 formerly the nature and character of the luminous discharges in vacuo 

 as obtained from my water-battery, as well as from the 400 insulated 

 cells of the nitric-acid battery (Phil. Mag. S. 4. vol. xx. p. 540) ; and 

 for this object I had several small vacuum-tubes constructed, about 

 3 inches long and 1 inch diameter ; in each of these, two metallic or 

 carbon balls, about -£• of an inch in diameter, were attached to the pla- 

 tinum wires, hermetically sealed in the tube about one inch apart. 



Fig. 1. 



I 



Each wire is protected by a glass tube as far as the ball ; the vacuum 

 is obtained by means of carbonic acid absorbed by caustic potassa, as 

 described in my former communications. 



6. The discharges in these tubes from an induction coil or from my 

 water-battery present nearly the same appearance, viz. a brilliant 

 luminosity surrounding the negative ball, generally without (fig. 2), 

 but sometimes with (fig. 3) a minute stratified discharge from the 

 positive. 



Fig. 2. Fig. 3. 



7. When the vacuum-tube is introduced into the circuit of 400 in- 

 sulated cells of the nitric-acid battery, the discharge at first always 

 assumes the form represented in fig. 2. This discharge, as well as those 

 from the induction coil and the water-battery, when examined, is found 

 to be very perceptibly intermittent, and will generally continue for some 

 time after the circuit has been completed. As the action of the bat- 

 tery improves, the luminous glow round the negative metallic ball 

 gradually increases in size, and in a few seconds the ball becomes 

 red-hot. This result I repeatedly obtained ; and in two instances 

 with tubes in which balls of aluminium, \ inch diameter, had been 



Fig. 4. 



9 o -~ 



^-t f-^. 



■/g^_ 



s~ 



inserted, the negative dropped from the wire into the tube in a molten 

 state, but leaving the positive ball with its original metallic lustre. 



