Mr. T. Gaffield on the Action of Sunlight on Glass. 517 



the specimens were placed, the sash being fastened together 

 by wooden pins, and placed on a nearly horizontal rough plate- 

 glass roof which received the direct rays of the sun during 

 the greater part of the day. In my next experiment I placed 

 some of the specimens directly on the rough plate glass. These 

 were carried on in summer and autumn. It was necessary to 

 make a different arrangement for winter, when the fall of snow 

 and the formation of ice might interfere with the full success of 

 the experiments. I concluded that the best size for specimens 

 was 4 by 2 inches; and I made some wooden boxes about § inch 

 deep, 4| inches wide, and of a length to fit the sills of windows 

 facing the south in the upper story of a Boston house. One of 

 the windows was three-sided, looking east, south, and west. In 

 these boxes (painted white) my specimens were placed in a 

 nearly horizontal position, side by side, and (after the loss of 

 a few pieces blown away by hurricanes and squalls) were se- 

 cured in their positions by cords of twine or slight copper wire 

 fastened at each end. I have arranged boxes on the roof in 

 front of this window also, some being of greater depth, in which 

 to place pieces of plate and rough plate about 1 inch in thick- 

 ness. All of these boxes are provided with covers, which are 

 placed over the glass and fastened by buttons on the occurrence 

 at any time of a snow-storm. At all other times the glass is 

 exposed. Holes in proper places in the boxes allow the rain 

 which falls to pass off easily. Every piece of glass is carefully 

 marked by a diamond in an upper corner with its name, and, if 

 necessary, with some abbreviation descriptive of the experiment 

 to which it belongs. This precaution is necessary to prevent 

 mistakes when a comparison is to be made of several kinds and 

 colours, of exposed and unexposed specimens. In all cases, 

 where an experiment has to be made with any description of 

 glass, the pieces (4 x 2 inches in size) must all be cut from the 

 same sheet, as there is frequently a slight difference in the shade 

 of sheets from the same factory, arising from the difficulty of 

 having the materials in their manufacture, and the circumstances 

 attending the melting, blowing, or casting precisely the same. 

 Two pieces or more should be laid aside to show the original co- 

 lour, and to compare with the others, and thus to show the changes 

 produced by exposure to sunlight for days, weeks, months, or years. 

 At the commencement of an experiment, say, for instance, the 

 exposure of white plate glass for from one to twelve months, a 

 neat paper box is provided, 4^ inches long, 2g inches deep, and 

 wide enough to hold fourteen pieces (two of the original colour 

 and twelve exposed specimens), and a description of the contents 

 is marked on its cover on an adhesive label. At the end of each 

 month a piece is withdrawn from exposure, carefully cleaned, 



