2 Lees, Thermal Conductivity of Salts. 



alcohol the thermal conductivity of which is known, and 

 the conductivity of the mixture of salt and liquid observed. 

 If it is greater than that of the liquid, a better, and if less, 

 a worse, conducting liquid is used, the operation being 

 repeated till no apparent change in the conductivity of the 

 liquid is produced by the addition of the salt. To prevent 

 chemical action going on during the test, the liquid is 

 previously saturated with the salt, and the thermal con- 

 ductivity of the saturated liquid either determined experi- 

 mentally or calculated from Jaeger's* results. 



The range of conductivities over which the method 

 can be strictly applied, is limited by the conductivities of 

 the liquids available, but it has been used over a wider 

 range in order to supply some approximate information 

 in a field in which our previous knowledge was nil. In 

 most cases only a rough approximation to equality of 

 conductivity of liquid and salt has been made. 



The apparatus, which was that used by me in deter- 

 mining the variation of conductivity with temperature, 

 has been described elsewhere in detail,f and will only be 

 described shortly here. It consisted of two discs of copper, 

 4 cm. in diameter and '3 cm. thick, cemented to opposite 

 surfaces of a glass plate of the same diameter and "281 cm. 

 thick. To the free surface of one copper disc, a flat 

 spiral coil of insulated platinoid wire was attached, and 

 the other free surface rested on a horizontal ring of ebonite 

 •326 cm. thick, 3*8 cm. internal and 7 cm. external 

 diameter, supported on, and cemented to, a third copper 

 disc 7 cm. diameter and '3 cm. thick. This disc was 

 placed on the top of a metal box, through which water 

 circulated. The liquid, or mixture of salt and liquid 

 to be experimented on, was placed within the ring of 

 ebonite, and the smaller copper discs pressed onto it till 



* See Landolt und Bornstein, Tabelkn. 

 t Phil. Trans., 1897. 



