HEADING COINS IN THE DARK. 113 



with these lights in succession would add to the 

 variety and wonder of the exhibition. The red 

 light might perhaps be procured in sufficient 

 quantity from the nitrate and other salts of stron- 

 tian ; but it would be difficult to obtain a blue 

 flame of sufficient intensity for the suitable illu- 

 mination of a large room. Brilliant white lights, 

 however, might be used, having for screens glass 

 troughs containing a mass one or two inches thick 

 of a solution of the ammoniacal carbonate of 

 copper. This solution absorbs all the rays of 

 the spectrum but the blue, and the intensity of 

 the blue light thus produced would increase in 

 the same proportion as the white light employed. 

 Amongst the numerous experiments with which 

 science astonishes and sometimes even strikes 

 terror into the ignorant, there is none more calcu- 

 lated to produce this effect than that of displaying 

 to the eye in absolute darkness the legend or 

 inscription upon a coin. To do this, take a silver 

 coin (I have always used an old one), and after 

 polishing the surface as much as possible, make 

 the parts of it which are raised rough by the 

 action of an acid, the parts not raised, or those 

 which are to be rendered darkest, retaining their 

 polish. If the coin thus prepared is placed upon 

 a mass of red-hot iron, and removed into a dark 

 room, the inscription upon it will become less 

 luminous than the rest, so that it may be dis- 

 tinctly read by the spectator. The mass of red- 

 hot iron should be concealed from the observer's 

 eye, both for the purpose of rendering the eye 

 fitter for observing the effect, and of removing 

 all doubt that the inscription is really read in the 

 dark, that is, without receiving any light, direct 

 i 



