540 Wisconsin Academy of Sciences, Arts, and Letters. 



ing to the tangential law of refraction of a disturbance cross- 

 ing the boundaiy of two media in which it is propagated with 

 conductivities ki and ks, 



K, tan. Q>i 



Kj, tclQ. «y^2 



The determination of the relative conductivities then resolves 

 itself into the measurement of the angles 4>x and ^o- In prac- 

 tice the isothermal line is determined bv the melting of a thin 

 layer of wax spread over the specimen. 



—.2) 



In applying this method to determining the relative conduc- 

 tivities of woods the following arrangement was used: Blocks 

 of wood about 8 cm. square and 2 cm. thick were cut in tri- 

 angular form and placed in a wooden frame with their fibers 

 parallel to AC, as shoAvn in the figure. These were covered on 

 the bottom and on the sides AC and BD with thick pieces of 

 asbestos to cut down the loss of heat. A uniform distribution 

 of heat along AB was obtained by pressing a strip of iron of 

 about the same dimensions as the edge of the block against the 

 wood, and heating this by means of a row of small gas flames. 

 The wax used to show the isothermal line was composed of par- 

 afiin mixed with a little turpentine. This mixture had a melt- 

 ing point of about 40° and gave a sharp line of demarcation 

 betsveen the melted and unmelted portions. To prevent the 

 wax soakinc: into the wood and also to insure a more uniform 

 radiation both blocks were covered mth a sheet of tinfoil on 

 which the melted wax was painted with a flat brush. This de- 



