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XLII. On the Stratification of Electric Light. 

 By the Rev. T. R. Robinson, F.R.S. ^j-c* 



IN a communication on the Stratification of Electric Light in 

 rarefied Media, which appeared in the Philosophical Magazine 

 for last July, Mr. Grove has described some facts which are in close 

 relation to the cause of that phsenomenon, of which the most 

 important is this — If in the circuit of an induction-machine of 

 which an exhausted vessel forms a part, there be an interruption 

 which is gradually lessened till sparks just pass it occasionally, 

 those sparks are blue, and have a single sharp sound ; if the 

 interval be still more diminished, they become yellow, burred, 

 and their sound is not so clear, but is attended with a slight 

 whiz. Now he finds that the blue sparks do not form strata 

 in the vacuum, but that the yellow do, so that by regulating 

 the distance he can produce them or not at pleasure. He thinks 

 the blue are single, and the yellow double or multiple ; and finds 

 in this a proof of his former opinion, that these strata are caused 

 by some peculiar action of compound discharges. Within the 

 last few weeks he has developed these views more fully in a lec- 

 ture at the Royal Institution. 



Everything which comes from Mr. Grove bears the stamp of 

 sagacity and power ; and I read this paper with great interest ; 

 but circumstances prevented me till recently from repeating its 

 experiments. The results which I have obtained, show that its 

 leading inference cannot be received as universal ; for, in the cir- 

 cumstances of my experiments, IdXw&ys obtained strata when a spark 

 passed, whether long or short, blue or yellow. Any one versed 

 in these inquii'ies knows how much the strata are modified by 

 slight variations of apparatus, &c. ; and some such have probably 

 caused this discrepancy ; I will therefore describe mine in some 

 detail. 



The induction-machine which was mostly employed, consists 

 of six sectional coils (the gift of my friend Dr. Callan), each con- 

 taining 8000 feet of fine iron wire, not lapped, but insulated by 

 an elastic varnish. They are arranged, three on each of two 

 vertical primaries, having each 350 spires of No. 13 copper, and 

 cores of iron No. 17. Their internal terminals are below, con- 

 nected in the centre; and the current is passed collaterally 

 through the primaries, so that the external terminals of the ex- 

 treme coils have equal and opposite tensions. The condenser 

 has 100 square feet of charged surface, which can be used in 

 sections of 20 according to the battery power. The rheotome is 

 mercurialf; and the machine gives sparks of about one inch 

 for every Grove's cell used to excite it. A few words respecting 



* Communicatcil by the Author, 



t Thiit kiud of rheotome is the best which I have tried; giving spaxks 



