406 Scientific Proceedings, Royal Dublin Society. 
the coefficient of expansion of the third material, would be in- 
teresting data for the completion of my diagram. 
The same sort of construction would also afford us the latent: 
heat of fusion. But before entering into details in this subject I 
would wish to point out a rough method of obtaining an idea of 
the relative latent heats of the minerals. It is the common expe- 
rience of practical mineralogists that certain minerals are more 
“easily fusible ” than others; and yet in many cases when put to 
the test of the meldometer it is not always the more easily fusible 
which has the lower melting point. Thus, orthoclase which is 
only with difficulty fusible before the blow-pipe (5 on Von Kobell’s 
scale of fusibilities) melts, according to Joly, at 1175° (Cusack 
finds 1166° for adularia) ; whereas labradorite with melting point 
at 1229° is “ easily fusible at 3”? on the same scale. Now, this is 
in itself a very remarkable result. In the example here given, 
the conductivities of the two minerals cannot be very different, 
and the cleavage fragments introduced into the blow-pipe flame 
will be closely similar. What, then, is the proper explanation 
of these apparent discrepancies?! It seems to me that it may 
be looked for in the latent heats of the minerals. It must 
be remembered that the temperature of the blowpipe flame is 
not much higher than the melting points of the minerals con- 
cerned, that the loss of heat by radiation is enormous, and that 
the minerals themselves are all very bad conductors of heat. Under 
these circumstances the supply of heat to the interior of the 
mineral grain will be very slow. We have then, evidently, a very 
sensitive arrangement for detecting the rate of absorption of heat 
by the substance under examination. I therefore submit that 
rate of melting or “ fusibility ” has to do with latent heat as well 
as melting point. There is, in fact, the same sort of connexion 
between “rate of fusion” and “melting temperature,” as there is 
between “ diffusivity” and “ conductivity.”” We shall, in future, 
want two terms for what has hitherto been vaguely included under 
‘‘fusibility,’ in order to distinguish clearly between “ melting- 
point” and “rate of fusion.” 
There are, I fear, as yet no exact determinations of the 
rate of fusion for the silicates. The cooling curves, so commonly 
1Cf. J. Joly, ‘‘On the Determination of the Melting Points of Minerals,” 
Proc. Roy. Irish Acad., Ser. 111., vol. ii. (1891), p. 39. 
