124 TRANSPARENCY TO LIGHT. 



upon matter, we have the phenomena of transmission, of 

 reflection, of refraction, of colour, of polarisation, and of 

 vision, to engage our attention. 



A beam of white light falls upon a plate of colourless 

 glass, and it passes freely through it, losing but little of 

 its intensity ; the largest portion being lost by reflection 

 from the first surface upon which the light impinges. If 

 the glass is roughened by grinding, we lose more light 

 by absorption and by reflection from the asperities of 

 the roughened surface ; but if we cover that face with 

 any oleaginous fluid, as, for instance, turpentine, its 

 transparency is restored. We have thus direct proof 

 that transparency to light is due to molecular condition. 

 This may be most strikingly shown by an interesting 

 experiment of Sir David Brewster's : 



If a glass tube is filled with nitrous acid vapour, which 

 is of a dull red colour, it admits freely the passage of 

 the red and orange rays with some of the others, and, if 

 held upright in the sunshine, casts a red shadow on the 

 ground ; by gently warming it with a spirit-lamp, whilst 

 in this position, it acquires a much deeper and blacker 

 colour, and becomes almost impervious to any of the 

 rays of light ; but upon cooling it again recovers its 

 transparency. 



It has also been stated by the same exact experimen- 

 talist, that having brought a purple glass to a red heat, 

 its transparency was improved, so that it transmitted 

 green, yellow, and red rays, which it previously ab- 

 sorbed ; but the glass recovered its absorptive powers as 

 it cooled. A piece of yellowish-green glass lost its 

 transparency almost entirely by being heated. Native 

 yellow orpiment becomes blood-red upon being warmed, 

 when nearly all but the red rays are absorbed ; and pure 

 phosphorus, which is of a pale yellow colour, and trans- 

 mits freely all the coloured rays upon being melted, be- 

 comes very dark, and transmits no light. 





