518 Mr. T. Gaffield on the Action of Sunlight on Glass, 



and marked either with a diamond or by an adhesive label, and 

 placed in the box. As " order is heaven's first law/' it is pecu- 

 liarly necessary in all observations upon the actions of heaven's 

 brightest luminary. By observing the above directions much 

 time will be saved, and at the conclusion of an experiment every- 

 thing is in shape for exhibition to friends at home, or students at 

 a lecture-room. 



Of course a perfect arrangement could only be made when a 

 perfectly flat roof or platform in an open field could be provided 

 and the sunlight could act with full force during every hour and 

 minute of the day. But mine was sufficiently near this point to 

 show very interesting results. 



In one of my earliest experiments I kept a record of the 

 changes going on in the various kinds of glass, at first from day 

 to day, and afterwards from week to week and month to month. 

 The following is a specimen : — 



Kinds of glass. 



Original 

 colour 

 before 



exposure. 



Colour 

 after one 

 day's ex- 

 posure. 



Here were inserted 

 other columns to 

 show effect of ex- 

 posure for 2 and 4 

 days, for 1 , 2, and 

 3 weeks, for 1 and 

 2 months. 



Colour at 



end of 

 experiment 

 (3 months). 



French white plate. 

 English crown glass. 

 French white sheet. 

 Belgian sheet. 

 American sheet. 









I might give the names of all the different kinds of glass which 

 I have exposed to sunlight; but I refrain because, for the reason 

 given previously, I have found different specimens from the 

 same manufactory of plate, of crown, and of sheet glass some- 

 times to differ in shade, sometimes in result of exposure, and 

 sometimes in both. 



I will, however, give below (not naming the particular manu- 

 factories) a general description of my experiments in 1863, when 

 an exposure of thirty-three specimens for a few months in sum- 

 mer and autumn showed the following results. 



The first Table shows that twenty-four kinds were found to 

 be easily affected, and exhibits the time at which the change in 

 colour was first observed. The second Table shows nine kinds 

 which did not change in colour during the same time. 



