on the supposed Diathermancy of Rock- Salt. 427 



The tube containing the thermometers was then placed in 

 the freezing-mixture. 





Naked thermometer. 





Enclosed thermometer. 



After the 







gain in 





After the 







gain in 



lapse of 



22 minutes, 35°; 



temp. 



27° 



lapse of 22 minutes 



52°; 



temp. 10° 



>> 



23 



„ 16 



ii 



19 



i, 23 



ii 



40 



„ 12 



i> 



24 



,, 8 



„ 



8 



„ 24 



it 



30 



„ io 



t> 



25 



ii 5 



y> 



3 



25 





20 



i, io 



ii 



26 



„ 3 



„ 



2 



,, 26 



>> 



17 



3 



„ 



27 



i, o 



>• 



3 



„ 27 



>» 



15 



2 



After the 











After the 









lapse of 



5 



„ o 









lapse of 5 



„ 



7 



8 



ii 



5 



„ o 









5 



ii 



3 



4 



» 



5 



„ o 









5 



j» 



1 



2 



>» 



5 



o 









5 



>> 







1 



It will be noticed that the naked thermometer reached its 

 maximum temperature 71° C. in seven minutes, at which 

 time the enclosed thermometer registered 49° C. The former 

 remained stationary at 71° C. for two minutes, and then slowly 

 descended ; after the lapse of ten minutes from the time it 

 first reached its maximum, and seventeen minutes from the 

 commencement of the experiment, it had fallen 6° C. and now 

 registered 65° C. During this time the enclosed thermometer 

 steadily increased in temperature, and now registered its maxi- 

 mum (64° C), it having risen 15° C, while the naked ther- 

 mometer had fallen 6° C. This reverse action proves beyond 

 doubt that the heat incident on the bulb of the enclosed ther- 

 mometer had been radiated from the salt as an independent 

 source, and not diathermanously transmitted. 



The following experiment, though void of numerical value, 

 is still, I think, interesting, as a different source of heat is em- 

 ployed. The apparatus used is shown in fig. 3 : T is a test- 

 tube with foot, 8 inches long and 1^ inch internal diameter ; 

 t, a second tube passed into the first and held in position by 

 means of a cork, c (this inner tube is f of an inch internal 

 diameter, and stands half an inch higher than the outer one); 

 P, a pivot fixed in the inner tube to support the source of heat 

 — a round bar of hot copper 2 inches long and half an inch 

 cross section ; C, a movable German-silver cap open at both 

 ends and polished on both sides ; its smallest circumference 

 passes into the inner tube, and its larger circumference passes 

 into the outer one. One surface of a piece of white blotting- 

 paper is coated with a thin layer of melted white wax, care 

 being taken that the wax does not penetrate through ; when 

 this layer is dry, others are applied till the texture of the paper 

 is well filled. 



The experiment is performed as follows : — ■ 



