Properties of Bees' -wcuc and Lead Chloride. 139 



improbable ; since the condenser, both when being tested and 

 when not tested, was kept in an atmosphere kept partially dry 

 with sulphuric acid. 



Some fresh lead chloride was prepared in the same way as 

 before — that is, by precipitating from a solution of lead acetate 

 with a solution of common salt, and carefully washing the 

 precipitate several times with distilled water. But the new 

 condenser, made with the carbon box and plate, was found to 

 have a resistance of only two megohms at 16° C; and, unlike 

 what was experienced in the former case, the resistance in all 

 the experiments was now found to increase by electrifications. 

 This, however, is probably explained by the fact that, whereas 

 in the previous case thirty volts electromotive force was used, 

 now only one volt was employed ; so that decomposition 

 (whether of the lead chloride itself, or of the damp which after 

 decomposition may act on the chloride) was probably not pro- 

 duced. Some preliminary temperature-tests were now made; 

 and, in accordance with Wiedemann's results, I found that 

 the resistance diminished with elevation of temperature. 



The condenser was now r left in an atmosphere dried with 

 sulphuric acid from December 21st to January 7th, when it 

 was found that the resistance had increased to about 15*6 

 megohms at 70° C. A large number of measurements of the 

 conductivity at different temperatures, using the same oil-bath 

 for raising the temperature as is shown in fig. 1, were now 

 made. The different experiments gave results so nearly 

 agreeing that they may all be represented by the curve STV 

 (fig. 4), in which distances parallel to X represent tempe- 

 rature, the points and X corresponding to the tempera- 

 tures 0° and 100° C. respectively ; and distances parallel to 

 Y represent conductivity on such a scale that the point T, 

 corresponding to a temperature of 70° C, represents a re- 

 sistance of about 15 "6 megohms. The curve is approximately 

 logarithmic ; that is, the ratio of the difference of the 

 logarithms of the conductivities to the difference of tempera- 

 tures is approximately constant. 



Thinking that possibly the method previously employed for 

 making the lead chloride, by precipitating it from a solution of 

 lead acetate with a solution of common salt, may have intro- 

 duced traces of some salt of lead other than the chloride, the 

 following method was now employed for making another 

 supply of the chloride. From a clear solution of lead nitrate 

 a precipitate was formed with colourless hydrochloric acid, 

 and the precipitate well washed with distilled water. A third 

 condenser was now constructed, three small pieces of glass 

 0*225 centimetre thick being used to separate the carbon 



