552 SCIENCE PROGRESS 



violet energy in the quartz tube and uviol mercury vapour 

 lamps. This matter may be recurred to here because it aptly 

 illustrates the important consequences following the discovery of 

 a source unprecedentedly rich in some special form of energy. 



It has already been explained that the amount of ultra-violet 

 energy obtained from incandescent solids is in most cases but a 

 fraction of i per cent. Ladenburg, however, has calculated that 

 as much as 30 per cent, of the energy in the mercury spectrum 

 is located in the ultra-violet. 



It is therefore hardly surprising to find that these lamps 

 possess some remarkable properties. The influence of ultra- 

 violet light in exciting chemical action may now be realised on 

 an unprecedented scale. Their application to the sterilisation of 

 milk and water, for tanning leather and for therapeutic work, has 

 already attracted attention. 



But perhaps the most interesting instance of their unexpected 

 properties is in connection with the fading of colours. The 

 gradual deterioration of all but the most permanent colours when 

 exposed to bright sunlight is of course familiar and has long been 

 attributed to the ultra-violet rays. So well was this recognised 

 that the manufacturers of carpets and dyeing materials in the 

 north of Germany were in the habit of sending their materials 

 periodically to the sunny south where their permanence could 

 be tested by abundant sunlight. But the advent of the new 

 lamps appears to have changed all this. Being far richer than 

 normal sunlight in ultra-violet rays it enables manufacturers of 

 coloured materials not only to test their fabrics in their own 

 works independently of climatic conditions but to do so in a far 

 shorter time than was previously required. 



It may be mentioned that excess of ultra-violet light is known 

 to have a very prejudicial effect on the eyes. Special precautions 

 are therefore necessary when using these lamps and when they 

 are applied for ordinary lighting purposes the internal quartz 

 tube is covered by a substantial outer globe of thick ordinary 

 glass by which the ultra-violet rays are absorbed. 



Yet it should not be hastily concluded that ultra-violet rays 

 are necessarily inimical to health (as has sometimes been done). 

 The cleansing sanitary effects of sunlight have been expressed 

 in an Italian proverb, " Where the sun does not enter the doctor 

 comes " and it is now believed that this beneficial action is at 

 least partly due to the valuable sterilising action of the ultra- 



