On Electrical Dust Figures observed on Plate Glass. 43 



in the number of the capillary trunks, which are now three instead 

 of two ; and at the summit are loops, frequently to the number of 

 four or five. These often require to be injected to be rendered 

 visible, as in the ordinary state some of them are frequently 

 either empty or full of the serous parts of the blood. 



Kensington, December 1850. 



[To be coutinueil.] 



VI. On Electrical Dust Figures observed on Plate Glass. 



By J. A. Broun*. 



[With a Plate.] 



IN the year 1843, while observing the magnetic dip at Ma- 

 kerstoun, I noticed some remarkable figures upon the glass 

 door of the inclinometer-case : these figures had a beautifully 

 ramified structure ; some of the lines being continuous, and some 

 formed of series of successive points. I showed this curious 

 phjEuomenon to Professor J. D. Forbes shortly after I first no- 

 ticed it, and found that it was as new to him as to myself. I at 

 first supposed that it was a magnetical phsenomenon, but upon a 

 little fui-ther investigation I found similar figures upon the glass 

 of clock-cases in all azimuths. Had my other duties permitted, 

 I intended to have made a series of experiments in connexion 

 with these figures, but have always been prevented ; I now there- 

 fore offer the fact for the investigation of others, with such re- 

 marks as have occurred to me in the course of my limited exami- 

 nation of the subject. The figure fii'st seen I copied veiy care- 

 ftdly ; it is shown in Plate II. 



It will be seen that the lines are at times beautifully curved 

 and continuous ; they run up against one another, but never 

 overlap, and seldom if ever come into absolute contact. In some 

 cases the lines are very bold, about a hundi-edth of an inch in 

 breadth, in others they are barely visible. Between the conti- 

 nuous lines there is often the faintest and most delicate tracery. 

 The dotted curved lines are formed of small particles at various 

 distances. At times the lines are formed of small continuous 

 pieces with breaks ; and where the break occurs, there is gene- 

 rally a small offshoot, as if the continuation had been diverted 

 into another channel. In some of the figures the lines are almost 

 wholly dotted, and the interior of the more regularly foi-med 

 portions is frequently filled with dots without any arrangement. 



The variations of fin-nis are excessive. I have watched the 

 growth of the figures, which goes on at times very slowly ; often, 

 however, the principal outlines seem formed in a single day, and 

 afterwards the interiors of the figures are filled with others, the 

 particles of which they are formed becoming fainter and fainter. 



♦ Communicftted bv the Author. 



