394 
MR. E. H. GRIFFITHS OH THE VALUE OF 
was of as pure platinum as could be obtained. The length used was about 13 in., 
and the wire was so disposed that parts near to each other should have only a small 
difference of potential. This wire was covered with a coating of the amber varnish 
before referred to. Several coatings form an excellent insulating covering for such a 
wire, but we contented ourselves with a thin skin. The better the insulation between 
the wire and the water the slower the flow of heat across the insulator, and the 
greater the difference in temperature between the surfaces which it separates. 
If the insulation was not perfect we have little doubt but that it was sufficient. In 
order to throw more light on this point we decided to make a careful determinatioi; 
of the value of R, with the coil immersed in a pure paraffin (pentane), but othervfise 
under the same conditions as those of the experiment. We selected pentane (l) 
because it would be difficult to And a more perfect non-conductor ; (2) if pure, it 
would be easy to free the calorimeter from all traces by passing through it a cm’rent 
of warm,/lry air. The latter was an important consideration, for any substance such 
as ordinary impure paraffin might so alter the interior surfaces of the caloruneter as 
to change the stirring coefficient. 
On December 8, 1892, about 250 cub. centims. of pentane^ were introduced into 
the calorimeter, which had previously been dried with extreme care, and the resist¬ 
ance of the coil was taken at twelve different temperatures. A summary of the 
results is given at the end of this section (see p. 413), and a very slight increase in 
the resistance is observable, amounting to one j)art in 22,000. It is satisfactory to 
find that, whatever may be the eftect of conduction through the water, it is, in this 
case, so small as to be unimportant. 
The measurements of II were made when the coil was immersed and the water 
stirred in the same manner as during the experiments, hence it follows that if the 
water acted merely as a conductor it would in no way affect our results. 
Even if its conductivity changed with the temperature our values of R include the 
effects of such change, and the manner in which R is built up at each temperature is 
of no consequence. 
Again, if electrolysis takes place (of which, however, we have had no sign), polariza¬ 
tion must ensue. The best proof of the absence of polarization is the fact that 
when the storage and Clark cell circuits were reversed (and this was invariably 
done more than once during each J experiment) no readjustment of the rheochord 
was required when using the platinum coil, although when using the manganine coil 
a cousiderable readjustment was necessary. 
Had electrolysis occurred, it is almost certain that HNOg would have been 
formed. In our earlier experiments (both in the platinum and silver calorimeters), 
* Great difficulty was experienced in obtaining tliis specimen of pentane, and it was only tbrougli 
the kindness of Mr. Yeenon Harcouet that we were able to procure it. It was prepared under his own 
direction and was free from all impurities, except some trac(3S of tetrane and hexane. 
