Intelligence and Miscellaneous Articles. 157 



through a space traversed by such a discharge. In v. Babo's appa- 

 ratus, as well as in those of Siemens and Houzeau, the metallic 

 conductors are separated by glass and a stratum of air. By induc- 

 tive action of the charged metallic surfaces the intervening air 

 becomes charged with electricity oppositely upon its two sides ; 

 and simultaneously with the discharge of the metallic terminals 

 through the wire of the coil, a discharge takes place through the 

 air, not in the form of sparks, but diffusely, producing a glow of 

 purplish light, visible only in the dark. 



These apparatus succeed best with electricity of comparatively 

 low tension. In using the Holtz's electro-machine with them the 

 discharge is apt to occur chiefly in the form of sparks through the 

 air, or it may even traverse and perforate the glass, and the form of 

 the apparatus must be varied to give the best results. 



When the poles of the machine itself are separated to a sufficient 

 distance, the electricity passes between them either in the form of a 

 diffuse brush, spanning the whole interval, or with a very minute 

 brush upon the negative pole, and a glow upon the positive, the 

 intermediate s^Dace not being visibly luminous. This is the so-called 

 dark or silent discharge, exhibiting the phenomena of the electric 

 shadovv^ when suitable objects are interposed, as described in a for- 

 mer paper*. When this occurs the strong odour shows that a con- 

 siderable amount of the atmospheric oxygen is converted into ozone. 



If this discharge is made to take place in an inclosed space through 

 which air or oxygen can be driven, the ozonizing effect of the elec- 

 tricity is heightened and can be utilized. The apparatus which I 

 have employed, and which has afforded very satisfactory results, 

 consists of a straight glass tube about 20 centimetres long and having 

 an internal diameter of 2*5 centimetres, the two ends being stopped 

 with corks covered on the inner side with a thin coating of cement 

 to protect them from the action of the ozone. Through the axis of 

 each cork is inserted a glass tube of about 5 millimetres calibre, and 

 7 centimetres in length, having a branch tube inserted perpendi- 

 cularly at the middle and long enough to permit a rubber tube to be 

 slipped upon it. The outer ends of the tubes themselves are closely 

 stopped with corks, through which are passed straight, thick copper 

 wires carrying suitable terminals at their inner ends, and bent into 

 a ring at the others. They are fitted so as to make tight joints, but 

 to allow of motion in order to vary the distance between their inner 

 ends. One of these wires carries a small ball ; the other terminates 

 in a disk with rounded edge, set perpendicularly to the axis of the 

 lube, and so large as to leave an annular space of some two or three 

 millimetres breadth around it. The gas is admitted through one of 

 the branch tubes, and escapes from the other after having passed 

 through the whole length of the tube. 



lu ushig the apparatus the wires must be connected with the poles 



of the machine in such a manner that the disk becomes the negative 



terminal, as this arrangement gives tiie greatest degree of expansion 



and diffuseness to the current. On turning the machine, and 



* Silhman's JournrJ (II.) vol. xlix. p. 381, and (III.) vol. i. p. 437. 



