386 Messrs. W. Spottiswoode and J. F. Moulton. [June 16, 



the intermitten6 discharge ah extra is eminently calculated to produce 

 Crookes' green phosphorescence, and the effect produced on this 

 phosphorescence by the striae of the internal discharge proves to be 

 not only well marked, but even striking, and of this we now propose 

 'to give an account. 



In the second of the two memoirs above quoted, Section 14, we 

 studied " the effect of intermittent inductive action of an impulsive 

 type upon continuous discharges." In the present instance the instru- 

 mental disposition was not very different from that which was there 

 described. The two terminals of a Holtz machine were connected in 

 the usual way with the two terminals of the tube, so as to produce 

 a stratified discharge. A narrow strip" of tin-foil, or a wire, was 

 stretched along the tube opposite the column of striae, but not extend- 

 ing much beyond it. The reason for the latter precaution will be 

 mentioned below. The positive terminal of a second Holtz machine 

 (in practice we used for this purpose a Toppler machine) was connected 

 with the tin-foil, and the negative terminal with one (either) terminal 

 of the tube. An air-spark, or interval across which sparks could pass, 

 was interposed in the part of the circuit between the machine and the 

 tin-foil. The effect of this arrangement was similar to that described 

 in our first memoir, p. 170, viz., in the interval between two sparks 

 the tin-foil and tube became charged like a Leyden jar ; the tin-foil 

 feeing the outer coating, charged positively, and the gas inside serving 

 as the inner coating, charged negatively. When the spark passed 

 across the interval mentioned above, the jar (i.e., the tube) became 

 discharged, and the electricity previously held bound on the two 

 coatings was set free. 



When the first (say the "internal") machine was not working, or 

 when it was disconnected, i.e., when no regular discharge was passing 

 through the tube, then, whenever a spark passed at the second (or 

 " external ") machine, a negative discharge with its accompanying 

 Crookes' radiation took place from the inside of the tube next the 

 tin-foil, and the opposite side of the tube became covered with a sheet 

 of green phosphorescence (the tube being of German glass). The 

 length of the sheet was equal to, or very slightly greater than, that of 

 the tin-foil strip, while the breadth was measured by the angular 

 dispersion from the strip, as described in the second memoir. 



When, however, other things remaining as before, a discharge from 

 the internal machine was sent through the tube, and a good stratified 

 column was produced, it was found that the green phosphorescence was 

 entirely cut off from the parts of the tube opposite to the striae, while 

 on the parts opposite to the dark spaces it remained, in the form of phos- 

 phorescent rings, at least as brilliant as before. The experiment was 

 repeated with various tubes with various degrees of strength of current, 

 and with various densities of gas (produced by heating a chamber of 



