102 A j iTSjE^&E^EN^EOLLiEs OF SCIENCE 



barium sulphate or Bolognian phosphorus, as it was called, 

 it was thought that this might be a re-discovery of the 

 long-lost art of making perpetual lamps. But it is well 

 known that this substance loses its phosphorescent power 

 after being kept in the dark for some time, and that occa- 

 sional exposure to bright sun-light is one of the conditions 

 absolutely essential to its giving out any light at all. This 

 condition does not exist in a dark tomb. 



A few years ago phosphorescent salts of barium and 

 calcium were employed in the manufacture of what was 

 known as luminous paint. These materials shine in the 

 dark with brilliancy sufficient to enable the observer to 

 read words and numbers traced with them, but regular 

 exposure to the rays of the sun or some other bright light 

 is absolutely necessary to enable them to maintain their 

 efficiency. 



More recently it has been suggested that the ancients 

 may have been acquainted with some form of radio-active 

 matter like radium, and that this was the secret of the 

 lamps in question. It is far more likely, however, that the 

 reports of their perpetual lamps were based upon mere 

 errors of observation. 



The perpetual lamp is, in chemistry, the counterpart of 

 perpetual motion in mechanics both violate the funda- 

 mental principle of the conservation of energy. And just 

 as suggestions of impossible movements have been numer- 

 ous in the case of perpetual motion, so impossible devices 

 and constructions have been suggested in regard to perpet- 

 ual lamps. Prior to the development, or even the sugges- 

 tion of the law of the conservation of energy, it was believed 

 that it might be possible to find a liquid which would burn 

 without being consumed, and a wick which would feed the 



