<58 



Tioyal Svciefy : — ■ 



bla'jkened pith op.gnt to move away from the pith, and therefore 

 towards the liglit, when the pith is ilhiiiiinated. This inference 

 was submitted to the test of experiDient by means of an apparatus 

 represented in fig. 1 and constructed as follows : — A piece of elder- 

 pith 2-b centims. in length and 1*2 ceutim. in breadth, blackeni'd 

 on one side, was fastened by one end to the interior surface of the 

 bulb of an ordinary boiling-fiask (of about 200 cub. centims. 

 capacity) in such a manner that the free end of the pith extended 

 towards the middle of the bulb. A light glass rod \^ith a small 

 magnet on one end, and a disk of thin microscope-glass on the 



other end, was so suspended in the bulb that the glass disk could 

 be readily balanced in a position nearly parallel ^ith the surface 

 of the blackened pith, and a few millims. distant from it. The silk 

 fibre from which the glass rod was suspended hung from a fixed 

 arm at the upper end of a tube, the lower end of v.hich was 

 hermetically fastened into the neck of the flask. An elongation 

 of this tube (not shown in the figure) with a contraction for seal- 

 ing, served to connect the a})paiatus ^^ith the exLaust-tube of a 



