84 THE ORIGIN OF LIFE 



mixed here and there only with Torulse (Plate 10, 

 Fig. 57). 



This last series of experiments with colloidal 

 silica is an especially important one for three prin- 

 cipal reasons. In the first place, a pure product 

 has been employed rather than a varying commer- 

 cial article, so that others will be able, when making 

 use of it, with more certainty to verify my results. 

 Secondly, it has been shown that for this solution 

 the tubes seem to be most advantageously exposed 

 to actual sunlight inside a thick glass window, 

 rather than outside to mere diffuse daylight. In 

 this way they get the advantage of a warmer tem- 

 perature as well as of a brighter light, the actual 

 sunlight having most of its actinic rays cut off by 

 the thick window-glass, plus that of the tube itself ; 

 so that, where not of too long duration, it seems to 

 exert no harmful influence. 1 



While, thirdly, dealing with such a solution as 

 I have employed in this last series of experiments, 



1 A reference to the last communication of Sir Arthur Downes, 

 entitled " The Action of Sunlight and of Diffused Light on 

 Micro-organisms," Proceed, of Royal Society, 1886, will show 

 that this is not as much as it may seem to be at variance with 

 his experience. The amount of sunshine in the summer of 1910 

 was much less than usual. Perhaps had it not been for this, 

 the exposure of tubes at the south window for one to two 

 months might have been harmful. Further observations are 

 needed. 



