360 EXPERIMENTAL INVESTIGATIONS. 



4 inches diameter ; the length of the wires forming the coils 

 are (I give M. Ruhmkorff's measurements), stout wire, 30 

 metres long, 2 millimetres diameter, 200 convolutions ; fine 

 wire, 2,500 metres long, \ millimetre diameter, 10,000 con- 

 volutions. These measurements will only be taken as approxi- 

 mative, and indeed the exact size is immaterial to the 

 consideration of the experiments which I am about to detail. 

 I will not give my experiments in the order in which I made 

 them, as I should have to describe many fruitless ones, but I 

 will place first that which I consider the most important and 

 fundamental. 



ist. On the plate of a good air-pump was placed a silvered 

 copper plate, such as is ordinarily used for Daguerreotypes, the 

 polished silver surface being uppermost. A receiver, with a 

 rod passing through a collar of leathers, was used, and to the 

 lower extremity of this rod was affixed a steel needle, which 

 could thus be brought to any required distance from the silver 

 surface ; a vessel containing potassa fusa was suspended in 

 the receiver, and a bladder of hydrogen gas was attached to a 

 stop-cock, another orifice enabling me to pass atmospheric air 

 into the receiver in such quantities as might be required.* A 

 vacuum being made, hydrogen gas and air were allowed to 

 enter the receiver in very small quantities, so as to form an- 

 attenuated atmosphere of the mixed gas. There was no baro- 

 meter attached to my air-pump, but from separate experiments 

 I found the most efficient extent of rarefaction for my purpose 

 was that indicated by a barometric height of from half to 

 three-quarters of an inch of mercury ; and, except where other- 

 wise stated, a similarly attenuated medium was employed for 

 all the following experiments. 



Two small cells of the nitric acid battery, each plate 

 exposing four square inches of surface, were used to excite 

 the coil machine, and the discharge from the secondary coil 

 was taken between the steel point and the silver plate. The 

 distance between these was generally = O'l of an inch, but 

 this may be considerably varied. When the plate formed the 

 positive terminal, a dark circular stain of oxide rapidly formed 



* See a figure, and description of the apparatus at th$ end of this Paper, 



