EFFECTS OF HIGH TEMPERATURES 51 



that with this solution the aim should also be to use 

 only one to three drops of the dilute sodium sili- 

 cate to the ounce, instead of eight or twelve drops, 

 as I had been previously employing. This, as I 

 have now realised, was a distinct mistake, since 

 it, in the first place, enormously increased the 

 difficulty of finding the organisms, when not very 

 numerous, in the midst of the copious deposit pro- 

 duced; and, secondly, because, as I have since 

 found, only very small quantities of silica are 

 really necessary. 



I have, in fact, now come to the conclusion that 

 whatever specimen of sodium silicate is being em- 

 ployed for the preparation of the colourless solu- 

 tion, only such an amount should be used as, after 

 boiling for ten minutes, will yield almost no de- 

 posit when the fluid has cooled. This minute 

 amount of deposit will, in the course of a few 

 days, undergo a very slight increase. 



I have previously called attention to some of the 

 effects of different temperatures on these colour- 

 less solutions, and to differences produced when 

 the tubes employed were of uviol rather than of 

 ordinary soft soda glass. 1 



This colourless solution is, doubtless, completely 



1 The Evolution of Life, 1907, pp. 266, 274. 



