434 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES 01* SOLIDS 



It is evident, therefore, that to investigate the law of variation of conductivity 

 of a mixture with its constitution more closely, it is advisable to work with mixtures 

 of constituents which differ in conductivity to a greater extent. If we confine our- 

 selves to liquids, we are limited to ratios of conductivities of about one to three, and 

 we must turn to mixtures of solids, or of solids and liquids to get greater ratios. 

 In the case of mixture of solids with liquids which dissolve them, a difficulty arises 

 as to which conductivity of the material entering into solution ought to be used in 

 making the comparison that of the material when in the solid, or when in the 

 liquid state. 



[The results given in the previous table for solutions of sugar in water are almost 

 identical with those of mixtures of glycerine and water, so that it may be said that 

 a solution of sugar behaves as if it were a mixture of water and a liquid having the 

 same density as sugar and a thermal conductivity of '0007. I have, however, found, 

 by the method described by me in ' Manchester Memoirs,' vol. 42, No. 5 (1898), that 

 the thermal conductivity of the solid sugar was about '0012, so that from a know- 

 ledge of the thermal conductivity of a solid we can infer little or nothing as to how 

 it will behave when it enters into solution. Thus, JAGER, ' Wien. Ber.,' vol. 99, 

 p. 245 (1890), found that solutions of sodium chloride, potassium chloride, and zinc 

 sulphate of about the same strengths had conductivities nearly alike and slightly 

 less than that of water, whereas I have found (' Manchester Memoirs' as above) that 

 the first two salts in the solid state conduct eight or nine times as well as water, and 

 the latter a little better than water. The result for ammonia solution given in 

 the above table points in the same direction, so that it seems advisable, for the 

 present at least, to confine our attention to mixtures in which neither constituent 

 has changed its physical state. The further question, whether such mixtures should 

 be treated as physical mixtures or as chemical compounds, must be held over till 

 more information has accumulated. 13th June, 1898.] 



If. on the other hand, solids are mixed with liquids in which they are not soluble, 

 there is great difficulty in keeping the mixture experimented on uniform, and in 

 determining its exact constitution. 



These considerations point to the conclusion that mixtures of solids would be 

 most useful in leading to the discovery of any law. They have, however, the dis- 

 advantage of not being as readily made as mixtures of liquids, a disadvantage which 

 may be removed by using as one constituent a semi-solid like lard or vaseline. 



A series of experiments have, therefore, been made with the apparatus, fig. 11, 

 p. 419, on mixtures of one of these substances with various amounts of reduced iron, 

 marble, zinc sulphate, and sugar, in the form of powder, and the results are given in 

 the following table and curves : 



