Mr. T. Gaffield on the Action of Sunlight on Glass. 521 



not believe that there exists in commerce a single species of 

 glass that does not change its shade in the sunlight." As all 

 manufacturers are in the same category, it will do no harm for 

 me to repeat, as the result of my experiments, what Pelouze says, 

 and to affirm that a longer or shorter exposure to the direct 

 action of the sun's rays will probably change in some degree the 

 colour of all, or nearly all, kinds of window-glass. 



I subjoin an account of an experiment carried on for one year 

 with nine different kinds of glass. These represent plate, 

 crown, and cylinder glass, the manufactures of both hemispheres 

 and almost every shade and colour of what are called colourless 

 glasses. 



I name only the nationalities, and not the particular manufac- 

 tories of the glasses in any of my Tables. I have this infor- 

 mation recorded in my journal, and shall be happy to commu- 

 nicate it to any who may desire it. 



Memorandum of nine different kinds of Glass exposed from 

 January 12, 1866, to January 12, 1867. 



Kind of glass. 



French white plate. 

 German crystal plate. 

 English plate. 

 English crown. 

 Belgian sheet. 

 English sheet. 

 American crystal sheet. 



»» >» 



American ordinary 



Colour before exposure. Colour after exposure 



Bluish white. 

 Light green. 



Brownish yellow. 

 Dark green. 

 Light bluish white. 

 Lighter bluish white. 

 Bluish green. 



Yellowish colour. 

 Bluish tinge. 

 Yellowish green. 

 Light purplish colour. 

 Deep „ „ 



Brownish green. 

 Purplish white. 

 Light yellowish green. 

 No change. 



The colours named above are given from an observation of the 

 glass edgewise, by which one can see a body of colour 2 or 4 

 inches in depth, whereas the usual thickness of the glass varies 

 from T ^ to 5 of an inch, and shows its colour easily only by 

 placing a white curtain or paper behind it. 



I have tried several experiments showing the effect of sun- 

 light during each month and each season of the year. At the 

 end of the year, by the comparative depth of yellow or purple 

 colour produced in the various specimens, one can see the com- 

 parative actinic power of the rays during each month and season. 

 The results proved that the actinic effect increased from January 

 to July, and decreased after that month. The greatest effect 

 during any season was observed in the summer, the least in win- 

 ter ; and that in spring: and autumn was about alike, and midway 

 between that of summer and winter. 



Crystal or lead glass and a piece of optical glass, having pro- 

 Phil. Mag. S. 4. No. 233. Suppl. Vol. 34. 2 M 



