133 



UNDULATORY FORCES.-LIG HT. [rowes or TUI BLIOTKIC LIGHT. 



nearly the required form : they are then put into a cruci- 

 ble, tillr.i up wuli tine sand, so as to secure the exclusion 

 of atmospheric air, and exposed for two or three hours 

 to a bright red or white heat The crucible u to be well 

 covered, and allowed to cool before the charcoal is taken 

 out 



Hard eolct has been employed with considerable ad- 

 vantage by many persona. The objection to it is, that it 

 frequently scintillates, from the large quantity of iron 

 which is containi-d in it. This difficulty may be overcome 

 by ]>owdering the coke, then stirring it about with a 

 magnet so as to attract the iron, or else digesting it in 

 iiitro-uiuriatic acid, and washing very well before <li 

 Tin- powdered coke is then to be moistened with a little 

 syrup r coal-tar, and rammed into a cylindrical mould: 

 the bar which i< thus made must be ignited in a crucible 

 ;.sl with sand, in the same way as that already 

 described for the preparation of boxwood charcoal. 

 These cylinders will be found exceedingly hard, and they 

 give a light which is superior in whiteness and intensity 

 to that from any other kind of charcoal. If the electrodes 

 are soaked in a strong solution of common salt, and then 

 dried, they give a still more brilliant effect; and by 

 using a salt of copper, chloride of strontium, <fcc., we 

 obtain green, red, and other coloured flames. 



Again, it lias been noticed that the intensity of the 

 light is increased by removing the atmospheric air from 

 the vessel containing the electrodes, and effecting contact 

 in a rarefied atmosphere, or in a vacuum. The effects 

 which are produced in this manner are very remarkable ; 

 for the purity and intensity of the light are greater than 

 those from any other source. In the first place we find 

 that it simulates the light of the sun, in the circum- 

 stance of its affording a means of distinguishing the most 

 delicate tints of colour : blues, yellows, and whites, 

 wliich are not to be seen in a pure state by ordinary 

 artificial light, are recognisable by this mode of illumi- 

 nation as if they were seen by day. Again, when the 

 light is decomposed by a prism, we obtain a spectrum 

 which is similar to that afforded by the rays of the sun. 



The intensity of the light has been variously estimated. 

 That which was exhibited by Mr. Staite on the 30th of 

 May,- 1849, from the summit of one of the towers of the 

 Hungerford Suspension-bridge, was said by him to be 

 equal to 750 wax candles. Professor Grove, in experi- 

 menting with a battery of thirty cells, the platinum 

 being four inches by two, obtained a light which equalled 

 that of 1,444 wax candles ; and Dr. Letheby states, that 

 in some experiments which were made by Mr. Hearder, 

 of Plymouth, in the month of April, 1849, with a 

 Maynooth battery of eighty cells, each four inches square, 

 arranged in two sets of forty each the light, when con- 

 centrated by a parabolic mirror, and sent over the 

 country for a space of 6,490 yards, gave a light equal to 

 that of a candle at thirty feet distance; so that the 

 intensity of the focussed light was- equal to that of 

 301,401 candles. It is probable that Mr. Grove's 

 estimate is nearest to the truth, as great pains were 

 taken in his investigations to arrive at correct results. 



It is very natural that a light of so great brilliancy 

 should command a large share of public attention, and 

 should likewise be made the subject of frequent experi- 

 ment. Hence it is that Mr. Staite and others have often 

 exhibited it in London and elsewhere. When it was 

 xhnwn from the top of the Hungerford Suspension-bridge, 

 the light was sufficiently intense to illuminate the water- 

 frontage of Somerset House ; and when it was cast to the 

 opposite side of the river, it lighted up all the buildings 

 on the Surrey shore. Exhibitions of it have also been 

 made at the Hanover-square Rooms, from the top of the 

 Duke of York's Column, and from the portico of the 

 National Gallery; but the most marvellous illustration 

 of ita power was afforded by Mr. Hoarder, of Plymouth, 

 who placed the light at the top of the Devonport Column, 

 and first experimented with it at Trematon Castle, which 

 is distant about 18,266 feet ; and then at Bovisand, which 

 is 16,470 feet from the column. At the former place the 

 light was sufficiently strong to mark the time on the 

 seconds-hand of a small watch, and the walks of the castle 



were distinctly visible at a distance of half a mile; 

 besides which, the ivy-leaves over the gateway of the 

 building were plainly seen when the observer was sixty 

 feet away from them. Its intensity, says Mr. Tucker, 

 who reported upon it, was magnificently brilliant. At 

 Bovisand the light was sufficiently strong to cast a 

 shadow of objects on a yellow wall, and it was thought to 

 be about equal in intensity to that of the full moon when 

 at its meridian in a calm clear night. At that distance, 

 without the reflector, it looked like the planet Venus 

 when seen through a telescope. 



The following are the accounts given of these effects, as 

 seen at Bovisand and Trematon, by Messrs. Walker and 

 Tucker, the two gentlemen who were deputed by Mr. 

 Hearder to make the necessary observations. The 

 accounts are extracted from the 1'lymoutb Herald for 

 April 21st and May 5th, 1849. Mr. Walker writes 

 thus : 



" With regard to our observations at Bovisand. About 

 half-past eight o'clock we saw flashes and glimmerings of 

 a bluish light from the column at Devonport, which, to 

 all but myself, were unsatisfactory. The people thought 

 that something had gone wrong. The light was then, in 

 all probability, on the Trematon side of the column, the 

 column being directly between us and the light itself. 

 After waiting for some time, we finally saw the electric 

 light outshining all the other visible lights, and we scut 

 up a rocket or two to indicate our satisfaction. Our 

 personal shadows were projected upon a boat-house door 

 (painted yellow, and itself illuminated) by the electric 

 light, and pronounced equal to that of the full moon when 

 on the meridian in a calm clear night. 



" A candle (six to a pound) projected a similar shadow 

 upon the same door, illuminated in the same way, at a 

 distance of 30 feet from the person whoso shadow was 

 thrown upon the door. That is to say, the shadow pro- 

 jected by the electric light at a distance of 5,490 yards, 

 was just equal to the shadow projected by a single candle 

 at a distance of 10 yards. Consequently, their illumi- 

 nating powers were as 10 (squared) : 6490 (squared), or 

 as 1 : 301,401. This comparison is rather a ' stunning' 

 one ! I am of opinion that the electric light possesses a 

 space-penetrating power infinitely superior to any light 

 we con obtain by combustion, since it is free from all 

 carbonaceous or solid particles ! 



" The Breakwater light may probably be equal to twenty- 

 four Argand lamps, each equal to half-a-dozen candles ; 

 yet this light projected a shadow only slightly visible at 

 a distance of 600 yards ; while the electric light projected 

 shadows, at lucid intervals, strong enough for children 

 to make rabbits with their fingers upon a wall more than 

 500 yards distant from the light itself ; that is to say, 

 ten times farther than from a lighthouse that could only 

 throw a shadow 'faintly visible.' The electric light, as 

 seen in the open air, may be compared (as far as colour 

 is concerned) to that of the planet Venus when seen 

 through a telescope, or to the light of a glow-worm, or 

 to those brilliant flashes of light we sometimes see within 

 the tropics, from the waves when surcharged with phos- 

 phorescent matter." 



And Mr. Tucker's account is as follows : " Sir, A 

 variety of occupations have prevented my sooner in- 

 forming you of the effect produced here by the electric 

 light, on the evening of the 12th inst. You are awaru 

 that this castle is 18,266 feet distant to the N.N.W. 

 from the column at Devonport. The reflector which 

 you used was one from which the rays diverge. The 

 evening was very favourable ; but little breeze (E.N.E.) 

 was stirring, yet sufficient to blow off the smoke from the 

 town. The atmosphere was so clear that the Devonport 

 lamps were very distinctly visible, and the stars which 

 appeared shone very brightly ; but they were few, and 

 the moon had not yet arisen. The instant the light 

 shone, the lamps of Devonport were all but totally 

 eclipsed : as it fell upon a soil spread over the walls of 

 the keep, we immediately perceived that a shadow was 

 cast by the fingers of a hand upon the sail, by a twig of 

 ivy, and by the stem of an ivy-loaf. We could clearly 

 see what o'clock it was by our watches. I ascertained 



