554 



Messrs. G. J. Stoney and R. J. Moss [Feb. 22, 



pith is illuminated. This inference was submitted to the test of ex- 

 periment by means of an apparatus represented in fig. 1 and constructed 

 as follows : — A piece of elder-pith 2-5 centims. in length and 1'2 centim. 

 in breadth, blackened on one side, was fastened by one end to the interior 

 surface of the bulb of an ordinary boiling-flask (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 with a small magnet on one 

 end, and a disk of thin microscope-glass on the other end, was so sus- 



pended in the bulb that the glass disk could be readily balanced in a 

 position nearly parallel with the surface of the blackened pith, and a few 

 millims. distant from it. The silk fibre from which the glass rod was sus- 

 pended hung from a fixed arm at the upper end of a tube, the lower end 

 of which was hermetically fastened into the neck of the flask. An 

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

 sealing, served to connect the apparatus with the exhaust-tube of a 

 Sprengel pump. The pump was set in action, and occasionally the flame 

 of an ordinary gas-burner was held at a distance of about 10 centims. 

 from the blackened pith, while the microscope-glass was closely watched. 



