578 
DE. ANDEEWS ON THE CONTINUITY OE THE 
each thread. The screw is packed with care, and readily holds a pressure of 400 atmo- 
spheres or more. A similar end-piece attached to the upper flange carries the glass 
tube containing the gas to be compressed (fig. 7). The apparatus, before being screwed 
up, is filled with water, and the pressure is obtained by screwing the steel screw into the 
water *. 
In the compound apparatus (figs. 5 & 6) the internal arrangements are the same as 
in the simple form. A communication is established between the two sides of the appa- 
ratus through a b. It is indifferent which of the steel screws below is turned, as the 
pressure is immediately diffused through the interior of both copper tubes, and is 
applied through the moveable columns of mercury to the two gases to be compressed. 
Two screws are employed for the purpose of giving a greater command of pressure. In 
fig. 5 the apparatus is represented without any accessories. In fig. 6 the same appa- 
ratus is shown with the arrangements for maintaining the capillary tubes and the body 
of the apparatus itself at fixed temperatures. A rectangular brass case, closed before 
and behind with plate glass, surrounds each capillary tube, and allows it to be main- 
tained at any required temperature by the flow of a stream of water. In the figure, the 
arrangement for obtaining a current of heated water in the case of the carbonic-acid 
tube is shown. The body of the apparatus itself, as is shown in the figure, is enclosed 
in an external vessel of copper, which is filled with water at the temperature of the 
apartment. This latter arrrangement is essential when accurate observations are made. 
The temperature of the water surrounding the air-tube was made to coincide, as closely 
as possible, with that of the apartment, while the temperature of the water surrounding 
the carbonic-acid tube varied in different experiments from 13° C. to 48° C. In the ex- 
periments to be described in this communication, the mercury did not come into view in 
the capillary part of the air-tube till the pressure amounted to about forty atmospheres. 
The volumes of the air and of the carbonic acid were carefully read by a cathetometer, 
and the results could be relied on with certainty to less than 0 - 05 millim. The tempe- 
rature of the water around the carbonic-acid tube was ascertained by a thermometer 
carefully graduated by myself according to an arbitrary scale. This thermometer was 
one of a set of four, which I constructed some years ago, and which all agreed so closely 
in their indications, that the differences were found to be altogether insignificant when 
their readings were reduced to degrees. 
I have not attempted to deduce the actual pressure from the observed changes in 
the volume of the air in the air-tube. For this purpose it would be necessary to know 
with precision the deviations from the law of Mariotte exhibited by atmospheric air 
within the range of pressure employed in these experiments, and also the change of 
capacity in the capillary tube from internal pressure. In a future communication I hope 
to have an opportunity of considering this problem, which must be resolved rather by 
indirect than direct experiments. As regards the deviation of air from Mariotte’s law, 
* The first apparatus was constructed for me by Mr. J. Ctjmine, of Belfast, to whose rare mechanical skill 
and valuable suggestions I have been greatly indebted in the whole course of this difficult investigation. 
