SCIENCE AND ELECTRIC LIGHTING 485 



and which are passing along the tube are absorbed by the gas before they 

 can reach the coating on the walls. This is therefore only feebly excited 

 by the discharge. By means of a charcoal side-tube, cooled in liquid air, 

 the gas can be absorbed and thus removed from the tube. As the pressure 

 falls the distance the electrons can move without being interfered with by 

 the molecules of gas increases, until they are able to strike the luminescent 

 powder on the walls with increasing force. The powder is then seen to be 

 excited to a brilliant fluorescence, which stops as soon as the gas pressure 

 falls so low that no more electrons are present. If the charcoal is heated 

 again by removing the liquid air the sequence of happenings takes place 

 in the reverse order. 



Before leaving this interesting and important subject of fluorescence we 

 should like to show some of the colours obtained by exciting these powders 

 by a lamp from which the visible light has been removed by a special dark 

 glass, thus leaving the invisible ultra-violet light only to come through. 

 In order to show these effects in this large hall I have had the powders 

 applied to sheets of cardboard, which we will now place successively in the 

 beam of ultra-violet radiation. First of all here is an uncoated piece of 

 white cardboard which does not fluoresce. This shows how little visible 

 light comes from this lamp. Here is the powder with a reddish fluorescence 

 which we have just used to improve the colour of the mercury lamp. You 

 will observe how some of the powders continue to glow — or phosphoresce, 

 as we call it, when the beam of ultra-violet light is cut off. 



All this, I hope, shows that the resources of science are by no means 

 exhausted in helping us, without using any more electricity, to add the 

 missing colours to light given out by some vapours and gases, thus making 

 objects illuminated under such light look as they do in daylight. 



High-Pressure Mercury Lamps. 



But that does not finish the story of the mercury lamp and its possibilities. 

 In the lamps we have been examining so far, the mercury vapour is at a 

 relatively low pressure when the lamps are burning. When we speak of 

 the vapour being at a low or a high pressure we simply mean that there are 

 respectively a smaller or a larger number of molecules of the vapour filling 

 the space in the tube and through which the electricity passes. 



It was found four or five years ago that very great yields of light could be 

 obtained from the passage of electricity through mercury vapour, if the 

 pressure of the vapour were increased to about one atmosphere, as compared 

 with about t ^q of an atmosphere in the older types of lamp. Further- 

 more, research showed the way by which the higher pressure lamp could 

 be made of simple construction. The result has been the improved lighting 

 of hundreds of miles of streets in Great Britain. There are, in fact, some 

 20,000 street lighting posts now fitted with these lamps. I have one of 

 these lamps here. They are well known now, but I would like you to look 

 at this projection of the luminous part of one of these lamps. The naked 

 lamp is too bright and too small to see properly from a distance, but the 

 greatly reduced brilliance of the projected image gets over this. It should 

 be noted how the discharge concentrates itself in a sort of central cord of 

 luminous glow. 



Now the increased intensity of this central luminous cord is the result 

 of the electricity tending to confine itself to this narrow track, which it 

 makes for itself through the molecules. The greater the number of 

 molecules of vapour present (i.e. the higher the pressure) the narrower does 

 this luminous cord become for the same electric current passing. This, 



