456 
MR.. E. H. GRIFFITHS OX THE VALUE OF 
The above Table is deduced from “stirring” experiments Nos. 41 to 110. Th 
values of p and cr, obtained by repeating an experiment under the same conditions, 
rarely show any difference as great as ‘000004, and thus a difference from the mean 
value of ‘000002 may be regarded as our limit of experimental error. Now a change 
of two in the last digit would, when 5 Clark cells wnre used, cause a difference of less 
than 1 in 4000 in the resulting value of J ; but as the numbers in each column are 
obtained from independent experiments, and as all the columns are used in the final 
determination of J, it is probable that the mean resulting error is of very small 
dimensions. 
[Note by E. H. G. Added April, 1893, The statement that the loss or gain by 
radiation, convection, and conduction is, in our apparatus, a linear function of the 
difference in temperature may at first sight appear to differ from the conclusions 
arrived at by Dulong and Petit, Macearlane, and other observers, but I think 
that the contradiction is more apparent than real. The curve which shows the 
absolute loss or gain (fig. 8, p. 454) indicates that, if we had reduced the pressure 
to something under one-thousandth of a millimetre, the loss or gain by radiation, 
convection, and conduction might almost have been disregarded over our range, and 
hence it is evident that convection plays by far the greatest part at such pressures 
as those with wdrich we were working; and the wonderful experiments of Professor 
Dewar (the account of which had not been published when the preceding section 
was written) appear to place this conclusion on a firm foundation. Now Duloxg and 
Petit make the loss or gain by convection vary directly as the difference in tempera¬ 
ture when the density of the gas remains constant. In our apparatus the density 
of the air surrounding the calorimeter must have remained nearly constant through¬ 
out each experiment, and therefore our observations appear to corroborate, rather 
than to contradict, the conclusions arrived at by Dulong and Petit.] 
Section XIII. —The Method of Performing a J Experiment, 
The general method of procedure was as follows. 
Freshly distilled ether having been placed in the cooling tube, the temperature of 
the calorimeter was lowered until the resistance of the coil indicated a temperature 
of 10° C,, any remaining ether was then withdrawn and dried air passed through the 
cooling apparatus for about ten minutes. The aspirating portion of the apparatus 
was removed and the cooling tube cleaned by a small mop of absorbent cotton wool. 
The open end was closed by a long cork, and a metal tube placed over the whole of 
the projecting portion, the lower end of this tube being washed by the tank water. 
The storage circuit wms then completed and the rheostat adjusted until the 
balance with the Clark cells was obtained. About twenty minutes elapsed before the 
temperature rose to 86 E„j—the point at which we commenced our observations. 
During this interval the chronograph circuit was tested, and any final adjustment of 
the motor and external circuit completed. 
