Mr. Grove on the Electro-chemical Polarity of Gases. 501 



(I give M. RuhmkorfPs measurements) stout wire, 30 metres 

 long, 2 millimetres diameter, 200 convolutions ; fine wire, 2500 

 metres long, i millimetre diameter, 10,000 convolutions. These 

 measurements will only be taken as approximative, and indeed 

 the exact size is immaterial to the consideration of the experi- 

 ments which I am about to detail. I will not give my experi- 

 ments 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. 



1st. 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 coixld 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 stopcock, 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 barometer 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 otherwise stated, a similarly attenuated medium 

 was employed for all the following experiments. 



Two small cells of the nitric acid battery, each plate exposing 

 4 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 =0*1 of an inch, but this may be considerably 

 varied. When the plate formed the positive terminal, a dark 

 circular stain of oxide rapidly formed on the silver, presenting 

 in succession yellow, orange and blue tints, very similar to the 

 successive tints given by iodizing in the ordinary manner a 

 Daguerreotype plate. Upon the poles being reversed and tlie 

 plate made negative, this spot was entirely removed, and the plate 

 became perfectly clean, leaving, however, a dark, polished spot 

 occasioned by molecular disintegration, and therefore distinguish- 

 able from the remainder of the plate. 



The experiment was repeated a great many times, and with 

 varying proportions of gas, and I found that with ])rop()rtions 

 varying from equal volumes of hydrogen and air to those of one 

 volume of the former to two and a half of the latter, the experi- 



* See a figure und description of the apparatus at the end of this pii[)er. 



