Prof. Thomson on some remarkable effects of Lightning* 55 



The circumstances with reference to the probang, de- 

 scribed by Mr. Leitch, afford a remarkable illustration of 

 the well-known fact, that an electrical discharge, when effected 

 through the substance of a non-conducting (that is to say, a 

 'powerfully resisting) solid, shatters it without producing any 

 considerable elevation of its temperature, not leaving any 

 marks of combustion, if it be of an ordinary combustible ma- 

 terial such as wood. 



Dr. Robert Thomson, at my request, kindly undertook to 

 examine the paper removed from the wall of the farm-house, 

 and enclosed with his letter to me by Mr. Leitch, so as if 

 possible, by the application of chemical tests, to discover the 

 staining substance deposited on its surface. Mr. Leitch, in 

 his letter, had suggested that it would be worth while to try 

 whether this case is an example of the deposition of sulphur, 

 which Fusinieri believed he had discovered in similar circum- 

 stances. Accordingly tests for sulphur were applied, but 

 with entirely negative results. Stains presenting a similar 

 appearance had been sometimes observed on paper in the 

 neighbourhood of copper wires, through which powerful 

 discharges in experiments with the hydro-electric machine 

 had been passed ; and from this it was suggested that the 

 staining substance might have come from the bell-wires. Tests 

 for copper were accordingly applied, and the results were most 

 satisfactory. The front of the paper was scraped in different 

 places, so as to remove some of the pigment in powder, 

 and portions of the powder from the stained and from the not 

 stained parts were separately examined. The presence of 

 black oxide of copper in the former was readily made manifest 

 by the ordinary tests ; in the latter no traces of copper could 

 be discovered. The back of the paper presented a green 

 tint, having been torn from a wall which has probably been 

 painted with Scheele's green ; and matter scraped away from 

 any part of the back was found to contain copper. Since, 

 however, the stains in front were manifestly superficial, the 

 discoloration being entirely removed by scraping, and since 

 there was no appearance whatever of staining at the back of 

 the paper, nor of any effect of the electrical discharge, it was 

 impossible to attribute the stains to copper produced from 

 the Scheele's green on the wall below the paper. Dr. Thom- 

 son, therefore, considered the most probable explanation to 

 be, that the stains of black oxide of copper must have come 

 from the bell-wire. To ascertain how far this explanation 

 could be supported by the circumstances of the case, I wrote 

 to Mr. Leitch asking him for further particulars, especially 



