MEMOIR XXX.] ON BURNING GLASSES AND MIKK( KS. .j.jc, 



the solution are compelled to cross it again, and, if not 

 already exhausted, thus to act once more. The follow- 

 ing illustrations are examples of the kind : 



Two small bulbs of equal size containing chlorine 

 water were exposed to the rays of the sun ; behind one 

 of them a concave hemispherical mirror was placed, so 

 that the rays which had crossed the solution were com- 

 pelled to cross it again. The amount of oxygen set free 

 in this bulb was about one fourth greater than that in 

 the other. 



The same was repeated, the exposure being to the sky 

 light instead of the sun-rays. The quantity of oxygen 

 set free in the two bulbs was as 18 to 55. 



It might be supposed that part of this increased effect 

 is due to the rise of temperature, from the mirror ob- 

 structing radiation. To exert a cooling action the fol- 

 lowing modification was therefore tried. In a glass jar 

 (Fig. 97) full of quicksilver a half-inch bulb containing 

 chlorine water was placed in such a way that a small 

 portion of its surface, about one eighth of an inch in di- 

 ameter, projected above the surface of the liquid metal. 

 On this part the solar focus from a burning-glass was 

 thrown. The rays therefore gained access to the interior 

 of the bulb, and were thrown about in all directions, 

 crossing and recrossing the liquid in every way by the 

 numerous reflections they underwent the mercury, as it 

 applied itself to the outer surface of the glass, acting like 

 a spherical concave mirror, and, from its mass and high 

 conducting power, effectually keeping the temperature 

 down. The quantity of pxygen emitted in a given time 

 was measured. The same experiment was then repeated 

 with the bulb removed from the mercury. After the 

 close of the same time, on measuring the oxygen set free, 

 it was found that the reflecting action of the mercury 

 had nearly tripled the effect. 



FF 



