and other Minerals when exposed to Heat. 



27 



pected, proportional to the rapidity of the change of tempera- 

 ture, which of course would correspond to the period at which 

 the temperature was highest, but on the contrary rose gradually 

 to a maximum, when the tourmaline was about half way cooled 

 to the temperature of the apartment; then gradually diminishing, 

 redescended to zero when it reached that point. This remarkable 

 result M. Becquerel obtained by suspending the crystal hori- 

 zontally by a fibre of silk under a glass cover, the temperature 

 of the air in which he had the means of regulating ; he then ap- 

 plied to the extremities of the crystal, wires from the opposite 

 poles of a dry pile, and, counting the number of oscillations made 

 by the tourmaline, deduced the intensity. 



The form of the experiment which I have contrived, and 

 which bears out M. Becquerel's conclusions, gives the same re- 

 sults with great elegance and simplicity, without attempting to 

 indicate the precise temperature of the stone at any period, 

 which, whilst the heat of the medium in which it is placed 

 changes, can only be by M. Becquerel's experiment an approxi- 

 mation, since the interior of the crystal must at any moment 

 have a different temperature from its surface. 



I employed a simple form of Cou- 

 lomb's electrometer, which I construct- 

 ed for the purpose with little difficulty. 

 A flat shaped bottle, AB, having a wide 

 tubulature at C, was provided. Fitted 

 to the neck is a tube D, plugged at top 

 by a cork F, through which passes a 

 crooked wire f, for the purpose of re- 

 gulating a fibre of raw silk, supporting 

 the needle of gum-lac e, one end of 

 which is terminated by the disk g of 

 gilt paper. The object to be examined 

 is introduced through the tubulature 

 C, the disk having previously been 



d 2 



