336 



was accompanied with a peculiarly sharp disagreeable crack noise, as if 

 two extremely hard things had been struck together ; but no two of the 

 reports, when the spark was very long, appeared to my ear to be exactly 

 the same, some being a little louder or sharper than others. In ordinary 

 electric machine sparks, taken from the prime conductor with a ball 

 placed at a certain distance, the sounds are, I believe, uniformly the 

 same, and to my ear more distinct ; but such is not the case with the 

 sparks produced by this great induction coil, when they are long. It 

 appears as if they must be different also when they are short ; but my 

 ear failed to notice it, while the eyes of some other observers appeared 

 not to notice a difference of another kind in the sparks. 



This is the occasional difference of colour between the right and left 

 halves of the sparks produced by the induction helix, when thej^ are about 

 from three to five inches in length. Supposing an observer to stand in 

 front of the apparatus, the half of the spark to his left hand, coming 

 from the inside terminal, always exhibited more or less a bluish-white 

 light, similar to that of sparks produced by approaching some conduct- 

 ing substance towards the prime conductor of a common electric ma- 

 chine when in good working order ; but the half of the spark towards 

 his right hand, or outside terminal of the helix, had always a different 

 colour. It was a sort of orange-red or salmon-colour, and fainter than 

 the other, and less luminous,- — suggesting to a believer in the doctrine 

 of two electric fluids an essential difference in the colour of each, the 

 bluish- white being the proper colour of one electricity, the orange- red 

 or salmon-colour, the peculiar colour of the other electricity. 



I here merely indicate the difference of colour observed between the 

 different ends of the sparks produced by the secondary helix, without 

 proposing any theory to account for it. I state the fact as one I ob- 

 served, which indicated a characteristic difference between the electric 

 sparks produced by this helix and electric sparks produced by another 

 agency. 



If one carefully watched the sparks composed of a left half of 

 whitish-blue, and a right half of salmon-coloured light, they would see 

 very often the salmon-coloured light form a fringe, or rather a case, 

 to the other, extending itself towards the left, beyond the medial point, 

 up to, if not to the starting-place of the white spark ; which would in 

 cases of this kind pass, as it were, through the centre of the salmon-co- 

 loured spark to the place it issued from : yet the eye could not detect 

 a difference in the moments of departure of the sparks. The spark 

 thus appeared to be one composed of two colours ; and to me it ap- 

 peared to always start from the right point. To other observers it ap- 

 peared to pass from the left. Hence this apparent difference may be due 

 to peculiarity of vision, peoples' eyes having different sensibilities, like 

 their ears — a fact well known to astronomical observers. In every case 

 the duration of the spark may have been so short that it was nearly in- 

 stantaneous, though the impression of it on the eye might have endured 

 as long as any other flash of light of the same intensity. Thus, no 



