374 
MR. E. H. GRIFFITHS OH THE VALUE OF 
several were broken in the process. The external diameter of the glass tubes was 
14 millims., the interior diameter of the gold tubes 18 millims. ; thus the annulus of 
india-rubber was very thin. The lower ends of the glass tubes were turned outwards 
so as to nearly cover the lower end of the india-rubber, of which the small exposed 
surface was originally coated with gutta-percha; but (as explained elsewhere) this 
was removed after J 19, and replaced by amber varnish.* The india-rubber corks, 
where they appeared above the lid, were, from the commencement, coated with amber 
varnish. No diffusion appeared to take place through these corks, and, as other 
observers seem to have been troubled by such diffusion when using india-rubber, it 
is probable that the coating of amber varnish (which adheres very firmly to the 
rubber) is an effective remedy. In any case it prevented any contact between the 
rubber and the contained water. We may thus assert that the water came into 
contact with the following substances only—gold, amber, glass, agate, gutta-percha 
(before experiment J 20) and (after experiment J 20) some hard shellac,! very small 
quantities of which were used to prevent slipping of the coil on the glass rods, the 
substance previously used for this purpose being gutta-percha. The platinum coil 
itself was coated with amber varnish, but it is possible, that owing to cracks in its 
coating caused by movements of the wire, the water had access to it in places. The 
small slabs of ebonite used at the junctions M and N (fig. 4, p. 396) were also 
covered with several layers of the same invaluable varnish. 
Section III. —The Method of Maintaining the Walls Surrounding the 
Calorimeter at a Constant Temperature. 
The necessity of maintaining at a constant temperature the walls of the chamber 
containing the calorimeter is so obvious that it seems unnecessary to dwell upon it. 
In our earlier experiments, the calorimeter was suspended in a copper vessel which 
was immersed in a large tank, and the uniformity of the temperature depended on 
the skill of the observer in adjusting the temperature of the tank water by altering 
the supply of hot and cold water admitted. The method adopted by us in 1891 and 
1892 worked automatically, and enabled ns to maintain a chamber at any temperature 
within the range of our experiments, whether that temperature was above or below 
that of the room. Briefly, the arrangement may be described as an enormous 
thermometer-bulb within which was the calorimeter. 
A section and plan, on a scale of are shown on Plate 3. The steel chamber, 
A, B, C (a very fine piece of work) was constructed by Messrs. Whitworth and Co. 
A detailed description of its various parts is unnecessary, as all particulars are given 
in the plate referred to. The annular space (coloured black) was filled with mercury. 
* See Note, p. 392, infra. 
t The shellac was melted on, and not deposited from its solution. A hard surface is rarely obtained 
by the latter method. 
