374 



H. M. Vernon 



exposed to every favourable purifying influence. It would be of 

 great interest to analysc a specimen of water colleeted at a con- 

 siderable depth beneatb the surface, to see if in tbis case also tbe 

 degree of purity was uo greater. Uufortunately bowever I was not 

 able to obtain one, 



Anotber proof tbat tbe limit of purity of tbe ammonia in the 

 water is tbat which is sometimes obtained by keeping water in 

 darkness, is afforded by tbe analyses of a specimen of open sea- 

 water t ken 5 km. from tbe shore. Tbis was kept in absolute dark- 

 ness, in a jar covered with black paper. 



From these figures we see tbat even after 40 days in darkness, 

 practically no purification was eftected. In faet, the water contained 

 more ammonia tban at first. Tbere was obviously some bacterial 

 action taking place in tbe water, bowever, as tbe variations in the 

 amount of free ammonia present are much too great to be referred 

 merely to experimental error. 



Tbe purification effected by keeping water in darkness, as tested 

 by its influence on the growtb of sea-urchin larvse, is very striking. 

 Tbe results obtained are colleeted in tbe following table. 





73 

 79 



82 



83 



84 



86 



118 



116 



119 



12S 



Conditions under wUch water had been kept 



days in semi-darkness 

 days in darkness . . 



(,seeded' water). . . . 

 (in jar containing alg«) 

 (after heatingtolOO°C.) 



(,seeded' watcri 



Amount, 



in purified 



water of 



free 

 NHa 



game 

 NH., 



.039 .098 

 .027 .100 



.007 

 .005 

 .007 

 .218 

 .020 

 .008 

 .007 

 .010 



.065 

 .059 

 .126 

 .125 

 .090 

 .068 

 .065 

 .097 



Amount, 

 in normal 

 Aquarium 



water of 



free 



.350 



game 

 NH?, 



.125 



,193 .143 



.251 



.129 



.074 \ .094 



»/o Va- 

 riation 

 in size 

 of larvae 



