VOLTOIETEIC EELATIONS OF OZONE. 
115 
foim represented in the figure. The liquid in the limbs d, e is hydrated sulphuric acid 
(HO, SO3), and it is by the changes in the level of this liquid that the alteration in the 
volume of the gas in dbc is determined. In order to make the necessary corrections for 
changes of temperature and pressure, during the interval between two observations, a 
vessel filled with dry air, of the same form and size as that employed in the experiment, 
was read along with it ; the reservoirs of both vessels being immersed in a large calori- 
meter, as shown in tig. 3. To the first of these vessels, we usually gave the name of 
primary vessel, and to the second that of auxiliary vessel. In order to correct for any 
shght diflerence in the size of the vessels, or in the diameters of the capillary tubes, 
simultaneous readings of both were made at difierent temperatures, and a coefficient 
thus determined, by means of which the indications of the two vessels could be after- 
wards accurately compared. When the reseiwoirs were large, the corrections so to be 
apphed were frequently less than the errors of observation. 
The extreme delicacy of this apparatus will be evident from the following consider- 
ations. If we take the case of a vessel with a large reservoir (fig. 1), the changes in 
volume of the contained gas, supposing the temperature to remain constant, will be 
nearly proportional to the changes of pressure indicated by a barometer filled with 
sulphuric acid. As the height of such a barometer, at the mean pressure of the 
atmosphere, would be about 6500 milhms., an alteration of 1 millim. in the difference 
of levels of the acid in the siphon tube {de^ fig. 1) would correspond to a change of volume 
of about g - Q^Q - o th of the entire gas ; but, as it was easy to read to 0-5 millim., or even to 
0-25 millim., the apparatus in this form enabled us to estimate a change of volume 
not exceeding one-half, or even one-fourth of that quantity. With a smaller reservoir 
(fig. 2), the indications of the apparatus were, it is true, not quite so delicate, and a 
careful set of comparative readings with the auxiliary vessel was always required ; but 
even here, a change of volume, amounting to not more than of the wliole, 
could be determined with certainty. 
The absolute change of volume of the gas, corresponding to a given change in the 
levels of the acid in the siphon tube (corrected in the first instance by the aid of the 
auxiliary vessel), was estimated in two ways ; first, by observing the change of level pro- 
duced by raising or lowering the temperature of the water in the calorimeter through a 
small number of degrees ; and secondly, by accurately determining, at the end of the 
experiment, the capacity of the reservoir and that of the capillary tube. 
The form of apparatus now described can only be employed when the entire change 
of volume of the gas does not amount, in the course of the experiment, to more than 
about one-tenth of the whole. When large changes of volume occur, the free end of 
the siphon tube must be hermetically sealed, so as to include a certain quantity of air, 
from whose subsequent change of volume that of the gas in the reservoir can be readily 
calculated. This modification of the apparatus we have found to be very convenient 
in experiments upon the action of the spark and silent discharge on the compound 
gases. 
