Vol. 4] Th dev. — Thermal Conductivities of Certain Schists. 211 



Methods Used by the Writer. 



The rock to be investigated was sawed or, where possible, 

 broken with a hammer, in such a manner as to give it a face 

 approximately parallel to the section wanted. This face was 

 then ground smooth and true, and coated evenly with a thin 

 layer of white wax. 



The wax used had a melting point of 63.6°, as determined 

 by the mercury bath method. Lower melting points, down to 

 38° C, can be secured at pleasure by adding turpentine in vary- 

 ing proportions. The wax was applied by dropping a few 

 shavings of it onto the previously heated face of the rock, tilting 

 the latter back and forth until a film of melted liquid covered 

 the whole upper surface, pouring off the excess, and then allow- 

 ing the whole to cool. The point of a hot copper wire was next 

 placed perpendicularly upon the prepared surface. As the heat 

 flowed outward radially from the point, a small, approximately 

 elliptical, area of the rock face became heated above the melting 

 point of the wax. 



The area of the ellipse increased more and more slowly, until 

 the integrated radiation and convection losses per unit time 

 became equal to the rate of flow across the last section of the 

 copper wire. The area of the growing ellipse will depend on 

 the one hand upon the conductivity and the thermal capacity 

 of the rock, and upon those properties of the surface which vary 

 the surface losses; and on the other hand, upon the time, the 

 temperature of the copper wire and of the air, and very largely 

 indeed upon the temperature of the rock. Other causes affect 

 it to an inconsiderable extent. 



The melted wax is drawn outward by its surface tension, 

 and, the point a being removed, it cools as shown in section in 

 fig. 1 (enlarged about 8 times). In this way, the 63.6° C. 

 isotherm is permanently fixed and may then be discussed and 

 measured at leisure. Usually 6 to 12 figures were thus fixed 

 on the same rock face before any measurements were taken. 



The apparatus used had to be so arranged that no radiant 

 energy capable of vitiating the results could reach the rock sur- 

 face. With this and several other ideas in mind, the following 



