1905.] Glass ly Natural Solar and other Radiations. 527 



I may perhaps be pardoned for quoting from my paper on the subject 

 the following passage, written 50 years ago.* 



" Some curious speculations arise from these facts. Should we be 

 able, by working under a vertical sun, and with every advantage of 

 cloudless sky, etc., to increase still more the length of our spectrum ? 

 Can we attain the limit of solar refrangible rays in this direction 1 Or 

 is it not more likely that there are emanating from the sun torrents of 

 rays which never approach the earth rays which, beating against the 

 upper stratum of the atmosphere, are themselves destroyed, but whose 

 vibrative energy is transmitted to us with increased wave-length and 

 lowered refrangibility, in the form of heat or light ? " 



Sunlight and radium both produce similar effects in these respects. 

 Their modes of action are known to be in the main very different ; but 

 it has been clearly shown that, in general, variation of time being dis- 

 regarded, what radium is capable of doing in the way of inducing 

 chemical change, ionising gases, producing phosphorescence, and 

 impressing a photographic plate, sunlight will also effect. 



[March 6. I am indebted to Professor McLeod for the following 

 historical note on the action of light on glass : " T. Gaffield, of Boston, 

 U.S., <B.A. Report,' 1872, Sect., pp. 3738. Bontemps, ' C.R.' 69, 

 1869, 10751078; 'Cosmos,' 6, 1870, 6675. In the ' C. R., 

 Bontemps presented to the Academy the results obtained by Gaffield, 

 together with some of his own. He refers to the experiments of 

 Faraday, which are described in 'Quart Jl. Sci.,' 19, 1825, 341. 

 Faraday describes the change to violet which took place in certain 

 glass after nine months' exposure to sunlight in London, and mentions 

 the colour of the window glass in houses in Blackfriars Bridge Road, 

 now pulled down, but which I well remember. Bontemps also 

 mentions work by Pelouze in 1867, described in ' C.R.' 64, pp. 53 66, 

 in which he attributes the yellow colour, produced in some glass, to 

 the formation of sulphides from the sodium sulphate by the action of 

 ferrous oxide, an explanation which Bontemps thinks improbable." 



Professor Judd called attention, when the paper was read, to the deep 

 colouration of the glass in the old greenhouses at Kew, in the parts 

 where it has been exposed to light. Glass coloured green with oxide of 

 iron for use in the forcing houses (as proposed by the late Robert Hunt) 

 is found by long exposure to sunlight to gradually lose this colour and 

 to become perfectly colourless. Subsequently a purple colour, due to 

 small admixture of manganese in the original frit, makes its appearance. 

 Visitors to the various houses in Kew Gardens can easily verify these 

 facts for themselves, as the most varied tints, from green to colourless 

 and purple, may be seen in different panes of glass, according to the 

 period at which they have been put in during repairs. 



* ' Journal of the Photographic Society of London,' vol. 2, p. 293. 



