432 Change of Heat Conductivity. 



thickness of the plate of thymol, and if k be its absolute heat 

 conductivity (g/cs), I found for the solid in two series of 

 experiments relative to A = -192 centim., & = 358/10 6 at 12°; 

 in six series of experiments for A = *153 centim., &=366/10 6 

 at 12° ; and finally, in eight series of experiments for A = '107 

 centim., &=354/10 6 at 11°. Variation of A was introduced 

 as a check on the method. Hence the mean value of the 

 absolute heat conductivity of solid thymol is at 12°, 

 & = 359/10 6 , from which the corresponding datum for ther- 

 mometric conductivity, /c, may be deduced by aid of the 

 above results for thermal capacity and density. Thus 

 *=1077/10 6 . 



In the case of liquid thymol at the same temperature, 

 although I made a very large number of experiments, I only 

 succeeded in obtaining six independent series faultlessly. 

 These were made relative to A = *107 centim., and showed 

 # = 313/10 8 at 13° and «' = 691/10 6 at 13°, as mean values. 



The data show that if the increase of thermal conductivity 

 of thymol, observed on passing at 13° from liquid to solid be 

 referred to solid thymol, the increment of absolute conductivity 

 Wc—k')lk\ is 13 per cent., while the increment of thermo- 

 metric conductivity is nearly 36 per cent. If the increments 

 be referred to the conductivity of liquid thymol at 13°, they 

 will necessarily be more striking, being 15 per cent, and 56 

 per cent, respectively. 



Now in the analytical theory of heat, it is the thermometric 

 conductivity which enters into the considerations of thermal 

 flow, and the marked effect which changes from solid to liquid 

 must necessarily produce is therefore obvious. 



The comparison of the undercooled liquid with the solid 

 may be deemed objectionable on the ground of possible 

 polymerism. To say nothing, however, of the enormous 

 complications of method introduced, little would be gained 

 from an application of pressure. For in my work on the 

 continuity of solid and liquid * I showed that the passage from 

 liquid to solid and back again, isothermally, by pressure, is 

 perhaps always accompanied by hysteresis. Thymol is simply 

 a convenient body in which the volume lag is pronounced, 

 and is observable at ordinary temperatures and pressures, 

 whereas in other substances the same phenomenon is exhibited 

 at higher temperatures and pressures. Indeed it seems idle 

 to ascribe to the molten liquid a more intimate relation to the 

 solid than the undercooled liquid, unless it be clearly specified 

 which this relation is. 



* American Journal, xlii. p. 125, 1891 ; cf. p. 140. 



