362 H. M. Vernon 



very carefully washed out })revious to its introduction, this vegetable 

 growth must bave arisen from the water itself. The comparatively 

 quick purification of the water must have been due to the rapid 

 increase of the originally very small quantities of vegetable growth 

 present. 



In another experiment, of a sirailar nature to the above, the 

 following results were obtained. 



Free NH^ Orgauic NH^ 



Original water .146 mgm. .114 mgm. 



After 13 days .139 .112 



- 24 - .048 .144 



- 36 - .008 .161 



- 50 - .030 .195 



In this case the rates of diminutiou of the free ammonia on the 

 one band, and of increase of the organic ammonia on the other, are 

 not nearly so rapid as in the previous instance. This was probably 

 due to the water baving contained less vegetable matter, for the 

 growth deposited on the sides of the jar was very much smaller in 

 amount. Microscopical examination showed it to consist eutirely of 

 diatoms, and a small amount of structureless greenish brown alga, 

 and not any of the bright green or purple alga noticed in the previous 

 instance. The temperature of the water during the experiment was 

 about 20" to 23° in the former instance. In the present case it was 

 about 25° for the first three weeks, then about 20° for the next fort- 

 night, and about 17° for the remainder of the time. As far as this 

 factor is concerned therefore, one would expect the rate of purification 

 to be more rapid in the second experiment than in the first, the 

 growth of the Vegetation being presumably more rapid at a higher 

 temperature. 



The results obtained on the growth of larvai in water which 

 had been exposed to diffuse light agreed with the chemical analyses 

 in proving that a marked purification of the water had taken place. 

 Thus in Exp. 73, the fertilised ova were allowed to develop in some 

 of the water obtained in the first of the above experiments, after 

 33 days exposure. They proved to be no less than 1\A% larger 

 than those grown in normal Aquarium water! However, the favourable 

 effeet was not so pronounced as one might be inclined to judge from 

 this increase in size, as only 35^ of the ova used, or about half 

 the normal number, developed into eight days larvai. In Exp. 129 

 on the other band, in which was used some of the water obtained 



