544 MR A. CRICHTON MITCHELL ON THE THERMAL CONDUCTIVITY 



out the bar. But, for a reason to be shortly stated, the results thus obtained 

 were not used. 



To this method of heating there is an objection. There is always a slight 

 smoking of the Bunsen burners, and this, together with moisture deposited at 

 the commencement of the heating, dulls the bright plated surface of the bar to 

 some extent. At the instance of Prof. Tait, who was surprised to find the 

 rates of cooling greater with the nickelised than with the plain bars, these 

 experiments were repeated in a form calculated to avoid any deposition of 

 moisture, and to avoid the smoking as far as possible. The bars were heated 

 to over 100° C. before a clear fire, and then as quickly as possible raised to a 

 high temperature over the row of Bunsen burners. The results of this set of 

 experiments have been used in the calculations. In Appendix I. a comparison 

 between the results of the two methods is given. 



The temperature of the bar during cooling was observed by means of a 

 particular thermometer, which was used almost exclusively (with an exception 

 already stated) for this purpose. As might be expected, the zero of this instru- 

 ment rose after repeated exposures to such high temperatures as it was employed 

 to indicate. But this error was carefully estimated. As already detailed, the 

 short bar during the progress of the statical experiment was placed near the 

 long bar, while its temperature (practically that of the surrounding air) was 

 recorded by a thoroughly trustworthy thermometer. The thermometer used 

 in the cooling experiment was simultaneously placed in one of the holes in the 

 short bar, and its readings compared with those of the other thermometer in 

 the bar. A continuous process of correction was thus established ; and by 

 always placing the thermometer in this position when not used for the cooling 

 experiment, the gradual change which the amount of the error underwent as 

 the experimental work proceeded was carefully noted. 



Prof. Tait, in the paper already referred to, pointed out the importance of 

 raising the short bar to a temperature considerably higher than that actually 

 required for the observation of the rate of cooling at any particular temperature. 

 The exact reason for this is, that although the mercury in the bulb of the 

 thermometer is heated almost at once, the column of mercury in the stem is 

 not so heated for some little time after, and that the cooling of the two regularly 

 together also does not ensue for some time. For example, in order to obtain 

 a determination of the rate of cooling at 200° C. (as accurately as the method 

 will allow), it is necessary to raise the bar to a temperature of 250° or 260°, 

 insert the thermometer as quickly as possible, and alloAv the bar to cool clown 

 through the temperature required. For a full experimental proof, see Trans. 

 It. S. A'., 1878, pp. 730, 731. This affords a complete explanation of the curious 

 result Forbes arrived at, viz., that the curve showing rate of cooling in terms 

 of temperature excess exhibited a point of flexure about 150° C. 



