442 M. G. Bontemps on some modifications in 



surface. I rightly attributed this colour to the presence of 

 manganese ; and, indeed, by suppressing the oxide of man- 

 ganese in the mixture, this effect no longer took place. Besides, 

 to prove that light had produced this colour, I took a pris- 

 matic ring recently made of glass containing manganese : I 

 broke it into two pieces, one of which, exposed to light during 

 a few weeks, became yellow ; and the other, kept shut up in a 

 drawer, was not at all altered in its whiteness. 



It is also known that some window-panes, especially the 

 Bohemian window-glass, take a light purple colour after having 

 been a long time under the influence of light. The same 

 effect is produced in window-glass or flint-glass containing a 

 small proportion of manganese, when they remain in the flat- 

 tening or annealing kilns long enough to produce incipient 

 devitrification ; in this case the interior of the glass becomes 

 opake white, whilst the outside takes a pink tint. 



I admit that some of the facts of colouring which I have 

 mentioned might be explained by reference to various degrees 

 of oxidation, and that manganese, for instance, loses part of its 

 oxygen when the glass passes from a purple to a yellow colour ; 

 but I doubt if this is sufficient to explain the phaenomena 

 which I shall call 'photogenic^ which take place when the glass 

 is in a solid state. 



3. Copper. 



Copper in its highest state of oxidation gives to glass quite 

 free from iron a sky-hlue colour, inclining more to green than 

 to purple, and in its lowest state of oxidation imparts a ruby 

 colour. In all times, as at the present day, red window-glass 

 has always been coloured by protoxide of copper; but it is 

 not very easy to obtain this colour, because it is not at all 

 fixed ; it must be seized at its proper time ; and this produc- 

 tion is the origin of a great many interesting and curious ob- 

 servations. When the red glass is in the proper state to be 

 blown, if it is ladled into water so as to effect a sudden cool- 

 ing, this \iVOi\nces yello'w-gree7i cullet; if this yellowish cullet 

 is heated to the point of liquefaction and cooled slowly, the 

 red colour will gradually show itself as the glass cools, 

 becoming of the finest ruby, inclining more to orange than to 

 purple : in some cases this colour is so delicate, that the cool- 

 ing resulting from the usual process of manufacture prevents 

 the manifestation of the red colour, and it is necessary to ex- 

 pose the manufactured piece of glass to the temperature of the 

 annealing kiln, in which case the red colour is seen to increase 

 gradually till it arrives at its greatest intensity : if the tempe- 

 rature of this kiln is too high, or if the ruby glass already 



