LIGHTHOUSE HI 



in darkness, two others were exposed in a western aspect behind glass, and the other 

 was turned to the south in the open air. 



The results were as follow: In the dark chlorine tube the colours were very 

 little altered, and would probably have been altered less had not the tube been fre- 

 quently exposed to light for the sake of examination. In the western tube, the 

 original grey and green wallflower papers became of a bright crimson, the blue 

 litmus bright red, and the brown rhubarb yellow. The whole of the chlorine had 

 apparently entered into combination with the colouring matters, for the yellow tint 

 of the gas had totally disappeared. In the southern tube the colour of the 

 chlorine could still be seen, the reddening action was less decided, and the 

 bleaching action was more powerfully evinced. The general result was that the 

 action of sunlight is less uniform than might have been expected in increasing the 

 bleaching power of chlorine, or while some tints rapidly disappeared under its action 

 assisted by light, other colours remained, in apparently the very same circumstances, 

 unaffected. 



Sulphurous add, if thoroughly dried, may be kept for months in contact with dry 

 colours without altering them ; under the influence of sunlight it however recovers to 

 some extent its bleaching power. 



Sulphuretted hydrogen acts as a weak acid, and readily as a bleacher when 

 moist, and becomes inactive in both respects if made dry and kept in darkness. 

 With the assistance of sunlight it recovers in no inconsiderable degree its bleaching 

 power. 



Oxygen is a well-known bleaching agent, but when dry its action upon colouring 

 matter in the dark is extremely slow. In sunlight, however, it recovers its bleaching 

 power. 



Carbonic acid, when dry in darkness, loses all power on colouring matter, but a 

 faint bleaching action is exerted by it under exposure to sunlight. 



Hydrogen is without any action when dry upon colours, but it acquires a slight 

 decolorising power when exposed to sunshine. 



'The general result,' concludes Dr. George "Wilson, 'of this inquiry, so far as 

 it has yet proceeded, is, that the bleaching gases, viz. chlorine, sulphurous acid, 

 sulphuretted hydrogen, and oxygen, lose nearly all their bleaching power, if dry and 

 in darkness, but all recover it, and chlorine in a most marked degree, by exposure to 

 sunlight.' 



All these experiments appear to show that the action of the solar rays on vegetable 

 colours is dependent upon the power possessed by one set of rays to aid in the 

 oxidation or chemical changes of the organic compound constituting the colouring 

 matter. The whole matter requires careful investigation. 



It is a proved fact, that colouring matters, either from the mineral or the vegetable 

 kingdoms, are much brighter when they are precipitated from their solutions in 

 bright sunshine, than if precipitated on a cloudy day or in the dark. It must 

 not be supposed that all the changes observed are due to chemical action ; there 

 can be no doubt but many are purely physical phenomena, that is, the result of mole- 

 cular change, without any chemical disturbance. 



3iIGHT CARBURETTED HVDKO&EN. Marsh-gas or fire-damp. 



ZiZGHT, ELECTRIC. See ELECTRIC LIGHT. 



LIGHTHOUSE. The importance of lights of great power and of a distinguish- 

 able character around our coasts is admitted by all. One of the noblest efforts of 

 humanity is certainly the construction of those guides to the mariners upon rocks 

 which exist in the tracks of ships, or upon dangerous shores and the mouths of har- 

 bours. This is not the place to enter largely upon any special description of the 

 lights which are adopted around our shores ; a brief account only will be given of some 

 of the more remarkable principles which have been introduced of late years by the 

 Trinity Board. 



The early lighthouses appear to have been illuminated by coal or wood fires con- 

 tained in ' chauffers.' The Isle of Man light was of this kind until 1816. The first 

 decided improvement was made by Argand, in 1784, who invented a lamp with a 

 circular wick, the flame being supplied by an external and internal current of air. 

 To make these lamps more effective for lighthouse illumination, and to prevent the 

 ray of light escaping on all sides, a reflector was added in 1780 by M^Lenoir; this 

 threw the light forward in parallel rays towards such points of the horizon as would 

 be useful to the mariner. Good reflectors increase the luminous effect of a lamp 

 about 400 times; this is the 'catoptric' system of lighting. When reflectors are 

 used, there is a certain quantity of light lost, and the ' dioptric ' or refracting system, 

 invented by the late M. Augustin Fresnel in 1822 is designed to obviate this effect to 

 some extent : the catadioptric ' system is a still further improvement, and acts both 

 by refraction and reflection. Lights of the first order havo an interior radius or focal 



