306 Royal Society : — 



3. Colourless transparent solids which exert a considerable pho- 

 tographic absorption, preserve their absorptive action with greater or 

 less intensity both in the liquid and in the gaseous states. 



Whether the compound is liquefied by heat or dissolved in water, 

 these conclusions respecting liquids are equally true. The perfect 

 permeability of water to the chemical rays, conjoined with the cir- 

 cumstance that in no instance does the process of solution seem to 

 interfere with the special action upon the incident rays of the sub- 

 stance dissolved, renders it practicable to submit to this test a great 

 number of bodies which it would otherwise be impossible to subject 

 to this species of experiment on account of the extreme difficulty of 

 obtaining them in crystals of sufficient size and limpidity. 



Glass vessels cannot be employed to contain the liquids during the 

 trial. Flint-glass, crown, hard white Bohemian, plate-glass, window- 

 sheet, and Faraday's optical glass, all, even in thin layers, shorten 

 the spectrum by from three-fifths to four-fifths or even more of its 

 length. Mica produces a similar effect. Indeed, the only substance 

 which the author found could be employed with advantage is rock- 

 crystal cut into thin slices and polished. The value of this material 

 in researches upon the more refrangible end of the spectrum was 

 pointed out by Prof. Stokes and M. E. Becquerel several years ago. 

 In order to hold the liquids for experiment, a small trough was pre- 

 pared by cutting a notch in a thick plate of plate-glass, the sides 

 being completed by means of thin plates of quartz, which were pressed 

 against the ground surfaces of the plate-glass by the aid of elastic 

 bands of caoutchouc ; a stratum of liquid of 0*75 inch in depth was 

 thus obtained for each experiment. 



The substances which, after atmospheric air and certain other 

 gases, are most perfectly diactinic, are rock-crystal, ice, as well as 

 pure water, and white fluor-spar. Rock-salt is scarcely inferior to 

 them, if at all. Then follow various sulphates, including those of 

 baryta, and the hydrated sulphates of lime and magnesia, as well as 

 those of the alkalies. The carbonates of the alkalies and alkaline 

 earths, as also the phosphates, arseniates, and borates, are likewise 

 tolerably transparent, though saturated solutions of phosphoric and 

 arsenic acids exerted considerable absorbent power ; so also did those 

 of the alkalies, potash, and soda, possibly from the presence of a 

 trace of some foreign colouring matter, as those liquids had an ex- 

 tremely faint greenish tinge. 



The soluble fluorides, as well as the chlorides and bromides of the 

 metals of the alkalies and alkaline earths, are freely diactinic, but the 

 iodides are much less so, and exhibit certain peculiarities. All the 

 organic acids and their salts which were tried by the author exerted a 

 marked absorptive action upon the more refrangible rays. Amongst 

 those subjected to experiment were the oxalates, tartrates, acetates, 

 and citrates, those mentioned first in order having the greatest ab- 

 sorptive action. It is, however, much more difficult to obtain or- 

 ganic compounds in a state of purity sufficient to furnish trustworthy 

 results, than is the case with the salts of the inorganic acids. The 

 author, therefore, expresses himself with more reserve upon some of 



