114 
DE. AKDEEWS AND PEOFESSOE TAIT OX THE 
some of the results already obtained are of considerable interest, and will be refen’ed to 
in the present communication. ^ 
Before proceeding further, we must draw attention to the difference of action which, 
in many cases, we have found to exist between the spark, or spark discharge, and the 
glow, or silent discharge. When the former terms are employed in this paper, they 
indicate a succession of brilliant sparks between two fine platinum wires, usually at the 
distance of 20 millims, (O’ 8 inch) from each other, and hermetically sealed into the 
tube containing the gas under observation. This form of discharge was obtained by 
connecting the free end of one of the platinum -wires -with an insulating stand, pro-dded 
with a brass ball which was brought within a short distance of the prime conductor of 
an electrical machine in high order, while the free end of the other platinum -wire was in 
connexion with the ground. The silent discharge presented no visible character except 
a faint glow, not visible by daylight, at each metallic point, and was obtained by con- 
necting the first platinum wire, not with the insulator, but directly with the prime 
conductor. To avoid the mixtru’e of the “ brush” with the silent discharge, it was 
necessary to establish the connexion firmly both with the conductor and with the earth 
wire ; and, in some cases, where a full effect was required, the machine had to be turned 
very slowly. 
The electrical machine employed was a small plate one (18 inches in diameter), 
screwed down firmly to the floor of the apartment, opposite to an open fij-e. On the 
prolongation of the axis of the plate, a wheel, 6 inches in diameter, was fixed, from 
which a belt passed to an iron wheel, 40 inches in diameter, revolving in a wooden 
frame, which was also fastened to the floor. By this arrangement, the machine could 
be easily made to turn at the rate of 350 revolutions per minute. To maintain a regular 
and powerful stream of electricity at this rapid rate of motion, it was found necessary, 
in addition to the ordinary cushions, to hold with the hand against the plate a rubber 
covered with amalgam. When in ordinary working order, the machme gave above 600 
sparks per minute, and in decomposing water produced in the same time 0’0002 cub. 
cent, of the mixed gases*. 
The ordinary forms of eudiometrical apparatus were found to be wholly inapplicable 
to this inquiry. We failed in discovering, by their means, whether even a change of 
volume occurs, when ozone is produced from oxygen. To increase the difficulty, the 
experiments could not be carried on in preseirce of mercury or water, as the former is 
immediately attacked by ozone ; and the latter not only destroys it rapidly by contact, 
but introduces a disturbing cause, in the form of aqueous vapour, exceeding in general 
the whole effect to be measured. In the apparatus now to be described these difficulties 
were overcome, and very minute changes of volume determined with certainty. 
In Plate III. figs. I and 2, the vessel in which the oxygen was contained is represented 
of different forms. It consists of a cylindrical tube ha-^ ing two fine platinimi uires 
hermetically sealed in opposite sides, and terminating in a capillary tube cde^ of the 
* Eeports of the British Association for 1855, Trans, of Sect. p. 46. 
