REPORT OF THE COUNCIL. 581 



carried out. As the nitrates, and also the large excess of ammonia are 

 almost certainly of bacterial origin, some experiments have been made 

 on treatment with a solution of bleaching powder. This reduced the 

 ammonia to one-third and killed all the bacteria, but the method was 

 abandoned as it caused temporary discomfort to the anemones and 

 other invertebrates in the tank. Treatment by electrolysis is now 

 being tried and has given encouraging results. The ammonia is reduced 

 and all the bacteria killed without aft'ecting any other living organisms 

 present, whether fish or invertebrates, and samples of the water 

 removed in flasks while still smelling of hypochlorous acid gave rise to 

 an abundant growth of diatoms and green algae. There is a possible 

 commercial application of this process to the treatment of shell-fish 

 from sources open to contamination, and experiments on these lines 

 are being carried out. 



The dissolved oxygen has been determined from time to time, and 

 the water has been always found saturated with this gas, even when 

 the tank was cut off from the general circulation and run on aeration 

 alone. 



At Christmas there is always considerable difficulty in obtaining 

 food for the fish, and advantage was taken of this period of enforced 

 starvation to determine the rate of increase of the bacteria. A small 

 tank containing a rather large number of pollack was cut off from the 

 circulation and run on aeration alone. After three days, during which 

 the fish had no food, the number of bacteria was only 3200 per cubic 

 centimetre ; five hours after feeding the number had risen to 46,000, 

 and next day, after a second lot of food had been given, there were over 

 150,000 per cubic centimetre. This last number is probably much 

 higher than the average for the whole system. 



Colorimetric examination of the outside water has shown that the 

 alkalinity during the early part of the spring of the present year (1912) 

 was far less than during the previous late summer and autumn. 



Mr. F. J. Bridgman has been occupied in an investigation of the age 

 of plaice found in the western portion of the English Channel. The 

 otoliths of a large number of fishes obtained from the neighbourhood 

 of Plymouth and from the bays on the Devon coast to the east of Start 

 Point have been examined for this purpose. 



During the winter a lieport on the Natural History of the American 

 Slipper-Limpet {Crepidula fornicata) was prepared by Mr. J. H. Orton 

 for the Kent and Essex Sea Fisheries Committee. In preparing this 

 report it became evident that definite information as to the food of 

 Crepidula would lie valuable to the Kent and Essex oyster-farmers, 

 whose oyster-beds are being overrun with this animal. A careful 



