470 On Heat and Light as transmitted through Glass. 



the dew is deposited is the coldest side, and that light has no 

 influence whatsoever upon such a deposition. My experi- 

 ments with camphor tend to a similar inference with respect 

 to the deposition of crystals upon the sides of vessels contain- 

 ing this substance. How that surface which is presented to- 

 wards the sun (as in Professor Draper's experiments) becomes 

 the coldest, is a paradoxical problem yet to elucidate. 



(/) If paper which has been prepared for heliographic 

 purposes by a solution of nitrate of silver be partially covered 

 by a piece of crown window glass, and then exposed to the 

 direct rays of the sun, that part of the paper which has been 

 thus covered will be rendered darker after a few minutes' ex- 

 posure, than that which has been equally exposed, but un- 

 covered. 



The experiments which I made during the last summer 

 upon the effects of solar light upon paper prepared with ni- 

 trate of silver, indicated that the maximum depth of tint en- 

 sued from its transmission through ordinary unstained crown 

 window glass; that the next tint in intensity was produced 

 by the direct solar rays upon the uncovered, and perfectly 

 exposed surface of the paper. The third in intensity, and 

 almost equal to the second, was produced by the transmission 

 of the sun's rays through glass of a " Waterloo blue" colour. 



The fourth, through dark violet coloured glass. 



The fifth, through purple do. 



The sixth, through amber do. 



The seventh, through brown yellow do. 



The eighth, through dark green do. 



The ninthj through light olive green do. 



The tenth, through blood red do. 



The eleventh, through crimson do. 



The twelfth, through bright red do. 



From the ninth to the twelfth there was no very remark- 

 able effect produced by the solar rays after an hour and a 

 half's exposure. 



The crown glass, the blue, the violet, and the purple glasses 

 produced deep tints, with which none produced from the 

 other coloured glasses could be compared but as contrasts. 

 The colours here mentioned were the only colours tried. 



There are one or two other peculiarities about crown glass 

 which deserve notice. 



1st. Why should the colour of unannealed window glass 

 be much lighter and brighter than that of similar glass when 

 annealed ? 



2ndly. The specific gravity of a specimen of very pure 

 doubly-terminated quartz at 64° Fahrenheit was 2 '65 7 7. 



