Electrification associated with Dust-Clouds. 493 



of the results obtained by this mode of experimenting (PI. VII . 

 figs. 5, 6,7,8,9,10). 



No. 5 is formed by starch dust, 6 from sulphur, 7 from 

 boracic acid, 8 from aluminium, 9 that due to merely sifting 

 sulphur upon the charged plate. Here the particles which 

 were uncharged, simply lie as they fall, and are not directed 

 along definite lines as is the case with the particles from the 

 dust clouds. 10 is due to sulphur sprayed, and consequently 

 charged, upon a positively charged surface. 



The easiest way to obtain these figures is to raise the dust 

 in a kind of wash-bottle. A wide-mouth bottle has a cork 

 with two bent tubes fixed in it (fig. 4) . One of the tubes 



Fisr. 4. 



passes nearly to the bottom, and the other passes just below 

 the cork. Some powder is placed in the bottle, and a stream 

 of air sent in through A. The dust and air coming out at 

 B are strongly charged, and on directing the stream against a 

 charged plate very beautiful figures are obtained. The same 

 apparatus can also be used for demonstrating the charges 

 developed by raising dust in general, for by allowing the 

 stream to impinge upon an insulated conductor connected to 

 an electroscope, very strong charges can be obtained, even 

 in a small room, but it is very difficult to determine whether 

 the effect seen is due to the charge upon the air or upon the 

 dust. If the current of air supplied is a gentle one, then 

 the normal electrical effect will be seen, i. e. red lead will 

 give a negative charge, but if the current is a strong one,, 

 the main effect may be that due to the charge upon the air, 

 and red lead then apparently gives a positive charge, but 

 the charge really comes from the air which accompanies the 



