DISCOVERY 



161 



colours, such as those in the wings of moths and 

 butterflies, show distinct fading after some weeks of 

 exposure, though the fur of some animals has been 

 found to resist action for more than a year. 



It is believed that the chemical action inducing 

 fading of colours demands three chief conditions, 

 the presence of ultra-violet light and oxygen and 

 moisture. Thus if an object could be kept perfectly 

 dry and enclosed in a vacuum, there should be no 

 fading, but in practice this condition is rarely realisable. 

 While it is probable that visible light may also play a 

 subordinate part in causing fading of colour, it seems 

 to be well established that the effect is mainly due to 

 ultra-violet rays. Indeed, such sources as the quartz- 

 tube mercury vapour lamp have proved of great 



^;value in enabling dyers and carpet manufacturers to 

 test the permanency of their colour rapidly and 

 scientifically, without being dependent on the capricious 

 periods of sunshine characteristic of the British Isles. 

 In the United States a special form of testing lamp for 

 this purpose has been designed. 



The most hopeful method of avoiding fading, there- 

 fore, is to exclude ultra-violet rays so far as possible. 

 Sir Sidney Harmer has experimented with various 

 glasses, more or less opaque to such radiation. Un- 

 fortimately the glasses that are most successful in this 

 respect appear to have a distinct yellow coloration 

 which would interfere with the observation of the 

 colours of specimens — even if such glass could be used 

 in sufficient quantities to enclose large objects. Another 

 drawback is that the glass diminishes fading but does 

 not eliminate it. The process is slowed down but is 

 still cumulative, and, the pitiless destructive force con- 

 tinues. Sir Sidney Harmer 's experiments, however, 

 suggest another interesting conclusion — that the 

 " fading effect " of light from electric incandescent 

 lamps is distinctly less than that of diffused daylight, 

 and very considerably less than that of direct sunlight. 

 It is worth consideration, therefore, whether objects 

 having fugitive colours might be housed in rooms 

 illuminated solely by artificial light — possibly by 

 " artificial daylight " of the type described in the 

 writer's former article, which would enable colours to 

 be correctly revealed. 



Other chemical effects of ultra-\iolet light have 

 been applied for industrial purposes for the tanning 

 of leather and in processes of sterilisation where the 

 rays are designed to destroy injurious bacteria. But 

 perhaps their most interesting effect is to be found in 

 the phenomena generally described as "fluorescence," 



l;,'i.e. their conversion into visible light. By using an 

 arc between tungsten electrodes, a quartz-tube mercury 

 vapour lamp, or other source rich in ultra-violet rays, 

 in conjunction with a plate of Chance's special glass, we 

 can obtain ultra-violet light almost completely free 



from visible rays. This " dark beam " produced by a 

 quartz lens is itself invisible. But when it falls on 

 certain objects they exhibit " fluorescence " and 

 themselves become luminous. Many substances have 

 this property in a greater or lesser degree. Certain 

 forms of zinc sulphide fluoresce with a vivid green, 

 calcium sulphide with blue light ; other materials can 

 be found to give us red and other hues, so that it is 

 possible to paint with them a picture, indistinguishable 

 by visible rays, but glowing in natural colours when the 

 ultra-violet energy is substituted. 



Many crystals and precious gems have the same 

 property. At a meeting of the Illuminating Engineer- 

 ing Society it was recently shown that by this means 

 false stones could be distinguished from genuine ones, 

 Indian pearls from the cultured Japanese variety, and 

 South African diamonds from Brazilian ones. One 



Fig. 2. — SHOWING THE DISTRIBUTION OF ENERGY IN 

 SUNLIGHT AT THE BASE AND AT THE SUMMIT OF A 

 MOUNTAIN 14,000 FT. HIGH. 

 (Based on Laugley's experiments.) 

 Not only is the total radiation at the summit far greater, but the propor- 

 tion of the ultra-violet is much increased, owing to the diminished absorption 

 by the atmosphere. 



can also prepare fluorescing solutions, and it has been 

 suggested that by this means we could provide an 

 additional safeguard in the preparation of bank-notes. 

 They could be impregnated with fluorescing material 

 leaving no traces in visible light, but showing a 

 luminous pattern — a species of secret watermark — 

 under the ultra-violet rays. 



Fluorescence and Economical Lighting 



Finally, it may be asked whether the phenomena of 

 fluorescence could not be applied to produce a highly 

 efficient luminous source yielding light without heat. 

 The idea is of course familiar to scientists. A corre- 

 spondent of Discovery ' draws attention to an extract 

 from a work entitled What is Electricity? by John 

 Trowbridge, published as far back as 1S97, and men- 

 tioning some experiments of Ebert." This investigator 

 is quoted as having utilised an electric discharge in an 

 exhausted glass globe, coated with fluorescent materials, 

 ' Mr. C. G. Bishop. ' Ann. der Physik itnd Chemie, No. 9, 1894. 



