376 Prof. Draper oa the Chemical Actmiof Laght. 



less and transparent^ the focal point was made to fall on the 

 yellow protoxalate at the bottom; but the decomposition could, 

 not be carried any further ; the yellow precipitate merely tvu-ned 

 darker, and recovered its tint as soon as the rays were withdrawn. 



This, therefore, would show that a very brilliant ray cannot 

 carry the decomi)ositiou further than a feeble one acting for a 

 correspondingly longer period of time. '"C. 



The following experiments were made in March 1842, on con- 

 secutive days, so selected that a thermometer, the bulb of which 

 was blackened when exposed to the sun, stood at from 93° to 

 94°. The intensity of the hght is supi)oscd to have been nearly 

 uniform. They show the behaviour of the substances referred 

 to in the focus of the lens ; and with a view in some instances 

 of cutting off the calorific effect as far as might be, and limiting 

 the action to the more refrangible rays, the glass trough was 

 filled with a saturated solution of sulphate of copper, as described 

 before. This, however, -did not remo\e all the heat, for a piece 

 of brown paper placed in the focus instantly scorched. For 

 the sake of brevity, I shall call the focus when . the sulphate of • 

 copper was used, the blue focus ; and where no absorbing mediijjnv' 

 had been employed, the white focus. '^J ' ' 



Bed lead, in the white focus, fused into a yellow mass ; but in 

 the blue focus there was merely a yellow dot, occu])ying the 

 centre of the focal point. This is an interesting experiment; 

 for it shows, that though the oxides of lead are so easily reducible, 

 the utmost force of the solar ray cannot remove the last atom of . 

 oxygen, though it so readily removes the rest. ; 



' Puce-coloured deuf oxide of lead gave precisely the same resultfejr-i 

 the deoxidation stopping at the same point. i^" 



Red oxide of copper turns gray-black in the white focus, the 

 same action ensuing whether the rays have come through the 

 blue solution or not. This effect is undoubtedly due to increased 

 oxidation taking ])lace at the expense of the air; for if thiso.xide 

 be put at the bottom of a \ essel of water and the focus be con- 

 verged on it, it exhibits ;^io. |Ch^nge,,!tlip,. atmospheric oxygen 

 having now no access to it. .. ,. ;.. ,, ,,ii ii^ 



Red oxide of mercury, in the white focus, turns yellow, and 

 quickly volatilizes; in the blue focus it exliibits no change. - • 

 Tuiiystic acid, exposed to the white focus, from a yellow turns 

 to a Mliitish-gray, undergoing the same changes more slowly, 

 but in the same inanr^r j^ ,,^hp , |lj(l^> fpcu^. tiJeneath water it 

 exhibits no change. , ,,; ,,, .. M ' .K"'. 



itefli(yff/- turns brown and then volatilizes. • 



Chromafe of mercury, in the white focus, is decomposed with 

 production of green oxide of chromium. 



Deutoxide of tin turns gray-yellow in the white focuis ; in the 



