402 



H. M. Vernon 



open sea-water. Nevertheless, after drawing a rapid ciirrent of air 

 through aboiit a litre of this water, for the space of four liours, only 

 9^0 of the carbonio acid was driven off. In fact, most of the carbonic 

 acid excreted by the animals into the water enters into unstable 

 combination with the salts present in solution. Hence aeration can do 

 bnt little to remove it, whilst most of the harmful influence its presence 

 might exert on the animai life is prevented. Perhaps aeration may 

 have a favourable etfect in checking the growth of harmful bacteria 

 which multiply only when the oxygen tension of the water is low, 

 but upou this poiut nothing definite is known. The experiments on 

 the effects of exposure of water to sun and air on the one band, 

 and to the sun only on the other, tend to prove that just the reverse 

 is the case, and that aeration causes an increased production of 

 harmful bacteria. 



The observations made on the elfects of aeration of the water 

 on the growth of sea-urchin larva? bear out our theoretical conclusion. 

 In the first experiment given in the accompanying table, the water 

 was aerated by allowing it to fall as a fine spray through a height 

 of 1 .8 metres. The process was repeated twice, but larvjt' grown in 

 the water thus aerated proved to be A.2% smaller than the normal. 

 This was probably due to some unknown error. In the next three 

 experiments, the water used was aerated by shaking it up violently 

 in a large flask frora time to time, over periods of several hours. 

 The larva? are on an average 2.3^ larger than the normal; hence 

 it is possible that excessive aeration may have a slightly favourable 

 ettect, though if the mean of the four experiments made be taken, 

 the positive influence averages only .05^, a negligeable amount. 

 Also as a mean of the four experiments, 68^ of the fertilised ova 

 reached the larvai stage, or practically the same as in normal 

 water. 



