1902.] 



On the Formation of Defin ite Dust-figures. 



285 



is reduced by exhausting the value of the source. A low tempera- 

 ture reduces the value of the injury current by checking the process 

 upon which it depends. Both extremes, then, reduce the value of the 

 current, but by quite different means. This being so, there will be a 

 mean temperature around which these two effects are balanced ; at 

 which the value of the source decreases less rapidly than at the higher 

 temperature, and the value of the diffusion process is greater than at 

 the lower temperature, at which, therefore, the injury E.M.F. is 

 best maintained. This consideration led to the systematic study of 

 the injury current at different steady temperatures, and the songht-for 

 point of best maintained E.M.F. was found to lie between 14° and 

 19° C. 



In the experiments hitherto considered, we have dealt with current, 

 no allowance being made for changes of resistance brought about by 

 altered temperature. The error, in some cases, was of no great 

 moment, in others, those, namely, where a lowering of temperature 

 gave an increased injury current ; a correction for altered resistance 

 would but have accentuated the point it was sought to establish. 

 But there are instances in which the error might be serious — and it 

 seemed, therefore, desirable throughout the inquiry to supplement 

 the photographic records by a series of observations, in which 

 measurements of potential by compensation should be taken at 

 frequent intervals. Such measurements have been undertaken by us, 

 and completely confirm the statements made above. 



1-;: . . 



On the Formation of Definite Figures by the Deposition of 

 Dust." By W. J. Russell, Ph.D., F.E.S. Beceived January 

 29— Bead February 19, 1903. 



(Abstract.) 



The author shows that when a plate of glass or other material is 

 slightly warmed and allowed to cool for 6 or 7 minutes in a dust- 

 laden atmosphere, a clear and definite figure is formed on the plate. 

 The figure is determined by the form of the plate on which it is 

 deposited. If a square plate is used then a simple cross is formed, 

 a ray of deposit proceeds from each corner of the plate to the centre. 

 If the plate be triangular, a ray again proceeds from each corner; and 

 with an octangular plate an eight-rayed star is formed. In every case the 

 number and position of the angles of the plate determine the form of 

 the figure. The dust generally used was that produced by burning 

 magnesium ribbon, but any fine dust acts in the same way and 

 produces the same figures. 



