Dust Figures produced by Electric Sparks. 729 



7. Creeping of Strive in tubes, both from and toivards the 



spark, was observed and sometimes it occurred both 

 ways simultaneously. 



8. Inclination of Strice in tubes produced by the spark was 



obtained by the simultaneous sounding of a tuning- 

 fork. 



Experimental Arrangements. 



The Surfaces. — The dust figures in the present work were 

 usually obtained on the surface of plate-glass placed hori- 

 zontally. This offered the advantage of a surface practically 

 plane, but being hvgroscopic it needed careful drying and 

 cleaning. Plates of steel and brass were occasionally tried 

 and, though capable of a high polish, often gave trouble by 

 a persistent though almost imperceptible curvature. 



Sometimes a cover of plate-glass was used to confine the 

 energy of the spark disturbance, thus securing cylindrical 

 instead of spherical radiation. In special cases tubes were 

 used instead of the plato, thus securing linear radiation in 

 the space under test. 



The Powders. — Twenty different powders were tested, but 

 most of them rejected for one fault or another. Thus, 

 aluminium and borax were too irregular in the shape of the 

 grains. The following were rejected on account of irregu- 

 larity in size of grains: — basic slag, bath-brick, black oxide 

 of iron, charcoal, cork filings, and pepper. A number of 

 powders were rejected because the separate grains clotted 

 together into a large cluster. This class comprised black- 

 lead, boric acid, corn-flour, flour, flowers of sulphur, French 

 chalk, plaster of Paris, rouge, and starch. Red lead was 

 difficult to shift owing to its high density and was prone to 

 clot. Iron filings are usually irregular as well as dense, but 

 their magnetic properties seemed helpful and some figures 

 were obtained with them. 



Lycopodium spores seemed by far the best powder to use 

 for these figures. The separate spores are almost identical 

 in size, nearly spherical in shape, and show only a very 

 slight tendency to adhere together. To dust the surface, the 

 lycopodium was well dried, a cloud of it thrown from a 

 pepper-box, and then the clean, dry plate passed into the 

 cloud, after the subsidence of any stray clots which might 

 be initially present. The repetition of this process of dusting 

 for five or six times usually gave a sufficient sprinkling to 

 form the basis of a dust figure. Because of its excellence 

 lycopodium was used practically throughout. 



The Sparks, — The electric sparks used for producing the 



