164 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
• 
ductivity may be supposed to converge, for a march from the ex- 
tremes of high values of conductivity possessed by metals or by sub- 
stances of positive temperature-coefficients to the extremes of low 
values of conductivity possessed by electrolytes or by substances of 
negative temperature-coefficients can hardly be supposed possible with 
the exclusion of the zero temperature-coefficient. 
The point which I am endeavoring to make becomes even of greater 
importance if we associate with metallic conductivity the correlative 
property of optic opacity. Relations between light and electricity have 
long been investigated, and many curious experimental facts are known. 
Maxwell's electro-magnetic theory of light furnishes a theoretical basis 
for the fact that all true conductors must be exceedingly opaque. 
Looking for a special application of this general principle it appears 
that solid metals, no matter how high the temperature to which they 
are heated, retain positive values for the tempera cure-coefficient. 
Similarly, Govi's 1 careful experiments prove beyond a doubt that solid 
metals, even in extreme states of red heat, remain opaque. The case of: 
liquid metals is by no means so definitely established ; and however 
uncertain and indefinite the evidence, the questions relative to possible 
transparency of liquid metals at very high temperatures is an open one. 2 
Considered from an electrical point of view, the increase of resistance 
of a metal from low temperature to the highest attainable, accompanied, 
as it is, by a diminution of the temperature-coefficient, points more em- 
phatically to an ultimate occurrence of optical transparency after the 
metal has passed from the solid into the fused state. Finally, inas- 
much as optical transparency may be considered as having been reached 
at the critical temperature, it is to this state that the occurrence of the 
zero temperature-coefficient is to be referred. 
For the present I may state that the position to be taken with refer- 
ence to the importance of this paragraph depends solely upon whether 
or not the result underlying Figs. 26 and 27 is to be taken as the ex- 
pression of a law. I have ventured to accept it as such. The remainder 
of the text is an application of simple geometric methods. I am nolj 
conscious of having forced any point, and the equation between f(Q) 
and /'(()) :/(0), at which I finally arrive (page 157), follows as an imme- 
diate inference. If in this equation either the first or second of these 
quantities be made zero, or if, in other words, the line be prolonged in 
a negative direction, the predictions of the line as a whole agree withj 
the known electric behavior of metallic conductors, and with the known 
electrical behavior of electrolytic conductors, and furthermore suggest 
the possible occurrence of an intermediate class of conductors, such that 
the passage from metallic to electrolytic conduction may be made conrj 
tinuously. 
1 Govi : C. R., vol. 85, 1877, p. 699; Secchi : Ibid., vol. 64, 1867, p. 778. 
"W. Ramsay: Cbem, News, vol. 55, 1887, pp. 104,175; Turner: Ibid., p. 163. 
(818) 
