Nov. 



21, 



18S9] 



^^■llls, gave a spark about 8 inches long over the excited glass 

 ^^Bbe ; and since the Leyden-jar, with a charge much lower than 

 that required to produce a spark \ inch long between the two 

 brass balls, was sufficient to give a spark about 13 inches long 

 over the excited glass tube ; it was at once seen that the length 

 of the spark over the excited glass tube, increases faster than the 

 intensity of the charge of the Leyden-jar. Of course the law 

 which connects the length of the spark over the excited glass 

 tube, with the intensity of the charge of the Leyden-jar, 

 can only be determined by experiment. It is, however, to 

 be noticed that, from the experiments of Harris and others, 

 the length of a spark in air of a Leyden-jar varies directly 

 with the intensity of the charge — that is, with the quantity 

 of electricity in the jar as measured by any such con- 

 trivance as the unit-jar. And further, that the length of the 

 spark over the excited glass tube depends (i) on the length of 

 the spark which the charge of the Leyden-jar can produce 

 between the l^-inch brass balls ; and also (2) on the degree of 

 electrification of the glass tube; and that both these two 

 quantities — namely, (i) and (2) — increase together. From these 

 considerations, I should expect to find that the length of the 

 spark over the excited glass tube increases in some way with the 

 square of the intensity of the charge of the Leyden-jar — that is, 

 with the square of the potential. 



' I dare say that the sparks over the excited glass tube, would 

 become very brilliant by using an induction coil to charge the 

 Leyden-jar. But to produce the maximum effect, the glass tube 

 should, I think, be lined, as in the following experiment, with 

 tin-filings instead of the tin-foil. 



A piece of hard German glass tube was taken, and one end 

 closed at the blow-pipe and the other end bordered to receive a 

 cork. After these operations, the tube was found to be just 

 2 feet 1% inches long ; the external diameter of the tube was 

 W inch, and the glass was 4^ inch thick. Next, the closed end 

 of the tube was filled with tin-filings to the height of 6 inches, 

 the filings having been condensed by tapping the end of the tube 

 on a piece of wood. A brass rod, with a knob at one end and 

 a screw having been cut on the other end, was screwed into a 

 cork which nicely fitted into the glass tube, and, by means of 

 the rod, the cork was thrust into the tube until it pressed upon 

 the tin-filings, and since the point of the rod was sharp and pro- 

 jected beyond the cork, the end of the rod entered a little way 

 into the tin-filings. The knob of the brass rod now stood just at 

 the mouth of the glass tube, and the mouth of the tube also con- 

 tained a cork through which the brass rod passed. Of the out- 

 side of the glass tube, the part surrounding the tin-filings was 

 painted over with lac varnish, and, as soon as it became suffi- 

 ciently sticky, a thin piece of tin-foil was wrapped around the 

 tube so as to cover the tin-filings, and no more. Lastly, the 

 remaining portion of the outside of the glass tube was painted 

 over with lac varnish. To charge this tubular Leyden-jar, it 

 was laid with the tinned end on one conductor and with the 

 knob of the brass rod on the other conductor of a Wimshurst 

 influence machine. I may mention, in passing, that the capacity 

 of this tubular Leyden-jar was surprisingly great in comparison 

 with its size ; thus showing that Leyden batteries, both cheap 

 and compact, can be made with the aid of glass tube and 

 metallic filings. The capacity is no doubt due, more or less, to 

 the uniform thinness of the glass, and to the close contact of the 

 tin-filings and the glass. The specific inductive capacity of hard 

 German glass does not seem to have been ascertained. But of 

 course, for the construction of Leydeu-jars, and also for the 

 plates of the Wimshurst machine, glass of the highest available 

 specific inductive capacity should be used. It may not be amiss 

 to remark that, owing to the high specific inductive capacity of 

 glass as compared with air, the efficiency of a Wimshurst 

 machine is probably much more increased by diminishing the 

 thickness of the stratum of air between the glass plates than by 

 diminishing the thickness of the plates. 



Now, the Leyden-tube produces a class of sparks which I do 

 not think have been shown by any other Leyden-jar. The 

 Leyden-tube was laid, as before mentioned, on the two con- 

 ductors of a Wimshurst influence machine, and the discharging 

 balls belonging to the conductors were set \ inch apart. These 

 two discharging balls were each i^ inch in diameter. On turn- 

 ing the handle of the machine, the Leyden-tube continued, of 

 course, to become charged and then to be discharged by the 

 s-inch spark between the discharging balls. But besides the 

 main spark between the discharging balls, little streams of elec- 

 tricity appeared along the glass tube, and extended away from 



NATURE 



59 



the tin-foil to a distance of i^ inch or more. These sparks were, 

 I think, best seen in a subdued daylight. They were very 

 numerous with each discharge of the tube ; I estimated the 

 I number of sparks in different discharges as varying between one 

 , and two dozens. The sparks were sinuous, very bright at the 

 tin-foil, and tapering away to nothing at the further end. Some 

 of the sparks, however, were not so bright as the others, and 

 rather ruddy ; they were probably inside the glass tube, and 

 coloured by the varnish on the tube. 



In the Leisure Hour, November 1888, p. 777 (56 Paternoster 

 Row), there is a photographic picture of a lightning-blaze, 

 wherein the bright ends of several of the flashes are seen to be 

 sitting upon what appears to be rock, and the flashes bear a 

 strong resemblance to the little sparks whose bright bases rest 

 upon the edge of the tin-foil. 



In the Leistire Hour, November 1886, p. 786, there is an- 

 other representation of a flash of lightning from a photograph. 

 In this instance, the flash is thick in the middle, but on ap- 

 proaching the earth, it tapers off to a fine point. Like as a river 

 may be only a small stream at its source and by gathering water 

 as it leads on to the sea, become a bulky stream at its mouth ; so 

 the sparks on the Leyden-tube gather up electricity from the 

 Leyden-tube, and so brighten away to the tin-foil. But in this 

 flash of lightning, the very reverse appears to take place. The 

 flash is greatly weakened before it reaches the earth, through a 

 transverse discharge to the air. For around the brighter portions 

 of the flash, the air is shining, and streamers are darting earth- 

 wards from the flash into the air. At the upper part of the flash, 

 there are also streamers acting manifestly as feeders from the 

 cloud to the flash. The flash rather resembles a long spark from 

 the prime conductor of an electric machine, than the spark of a 

 Leyden-jar ; but the prime conductor being metallic, can only 

 imperfectly represent the much lower conduction of a cloud. 



In the Leisure Hour, September 1889, p. 641, there is an 

 engraving from a photograph of the so-called ribbon-lightning. 

 This form of lightning is clearly produced by a succession of 

 flashes following along the same path, combined with some slight 

 motion given to the camera by the hand of the operator ; as 

 indeed is there pointed out. The question is. How comes it that 

 the flash so repeatedly passes along the same path ? The answer 

 there given is that suggested by Mr. Cowper Ranyard, "That ap- 

 parently the first flash would heat the air and slightly rarefy it, 

 leaving a path of least resistance, along which subsequent dis- 

 charges would flow as certainly as water follows the twists and 

 turns of a pipe." It seems to me, however, that a far more im- 

 portant cause for making a second flash to pass along the path of 

 its predecessor is to be found in the action of the transverse dis- 

 charge, whereby a tubular mass of air becomes electrified around 

 the path of the first flash ; and through the electrified air, the flash 

 readily passes, as previously shown. In the woodcut, the efful- 

 gence of the surrounding air and the streamers show that the 

 lightning was distributing electricity along its path. The trans- 

 verse discharge is perhaps never absent from the flash of lightning. 

 In Nature, vol. xl. p. 543, a flash of lightning which struck 

 a windmill, is described as " a mass or network of flame, which 

 threw off thousands of sparks like fireworks." 



The discharging balls of the Wimshurst machine were set one 

 inch apart, everything else remaining as before. The sparks 

 now extended along the glass tube to a distance of about 3J 

 inches from the tin-foil. The general character of the sparks was 

 the same as before, when the discharging balls were set half an 

 inch apart. 



The discharging balls were set i| inch apart. When the dis- 

 charge occurred, the sparks extended along the tube to about 54 

 inches from the tin-foil. The sparks were straighter, and not 

 nearly so numerous as when the discharging balls were set at 

 half an inch ; they were also very much brighter, but like the 

 others, they all tapered away to nothing. In this experiment, 

 the Leyden-tube was charged to about the highest potential that 

 the machine would give it ; and the matter was not any further 

 pursued. Reuben Phillips. 



I Bay View Terrace, Northam, Bideford, October 9. 



" Darwinism." 



What my "laborious essay " " distinctly professes to be " is, 



as its title-page announces, ' ' an additional %\xzz&%ViOX\. on the origin 



of species "; and this additional suggestion is forthwith stated to 



be that of '' another factor in the formation of species, which, 



