Conductivity in Tourmaline Crystals, 21 



It then occurred to the authors that it would be worth while 

 to measure the flux of heat and of electricity across a thin 

 wall of tourmaline, the crystal being cut for that purpose at 

 right angles to its crystallographic axis. For some months 

 no suitable crystal could be obtained ; but eventually, by the 

 kindness of Professor K Story Maskelyne, this difficulty was 

 removed, and a slice of opaque black tourmaline from the 

 Ural Mountains, which we will speak of as tourmaline " B," 

 was placed at our disposal. Its dimensions were 35*5 millims. 

 length by 24*4 millims. width ; and it varied in thickness from 

 2*5 to 2*14 millims. 



Two methods of measuring the flow of heat through such a 

 slice were suggested. One method, applicable only to a slice 

 at a constant temperature, is described, and the mathematical 

 solution of the case is given, in the Philosophical Magazine 

 for February 1878, by the second-named of the authors, under 

 the title " On a Method of Measuring the Absolute Thermal 

 Conductivities of Crystals " &c. The opportunity of applying 

 this method to the tourmaline has not, however, arrived. 



A second and simpler method of experimenting, and giving 

 results of qualitative value only, was suggested and carried 

 out by the first-named author. It involves the use of a 

 sort of reversible contact-thermometer. The crystal-slice 

 was fixed between two portions of a glass tube. In one 

 of these a weighed quantity of mercury was placed and a 

 thermometer. Into the other steam was blown, so that heat 

 passed upward through the crystal and warmed the mercury. 

 After taking a measurement in this position the apparatus 

 was inverted, the mercury being placed in the other tube, and 

 the steam directed into the tube formerly serving as the calo- 

 rimeter. The glass tube chosen was about 20 millims. in in- 

 ternal diameter, and about 1 millim. thick. It was divided 

 into two parts, the edges fused and thickened and then ground 

 flat, and very slightly smeared with a mixture of resin and 

 Canada balsam. The crystal slice was placed between them, 

 and they were bound together with strips of black caoutchouc. 

 This joint was perfectly watertight. After a number of expe- 

 riments in which the heat was made to pass through the crystal 

 first in one direction, then in the other, the crystal was taken 

 out, reversed in position between the two glass tubes, again 

 secured in its place, and a fresh series of experiments were 

 made. The condition of the crystal, as far as could be ascer- 



rnal curves being obtained for low temperatures by the melting of films 

 of cocoa-butter, and for higher temperatures by films of solid paraffins of 

 definite fusing-point. 



