SUPPLEMENT. 1235 



very severely felt in the Blackwater and its tributaries and 

 estuary, where the cold killed all the French oysters and a 

 great many natives that were left in exposed places, though 

 the natives laid in pits escaped. 



At Wyvenhoe, Messrs. Harvey attribute the fatal 

 effects of cold on the beds to the increased density of the 

 water ; but as the greatest density of this brackish water 

 near Mersea is at about 39 Fahr., it is difficult to see why 

 this should be the cause. Frost sometimes freezes the shells 

 together, and the oyster dies from starvation. Oysters have 

 been opened and found to contain the fish enveloped in a 

 lump of ice. In this state, though dead, it is perfectly good 

 to eat, if eaten at once, though when it is thawed the dead 

 animal becomes quickly putrid. In winter, after a thaw, 

 snow-water comes down the rivers and increases the volume 

 of fresh water, and thereby sometimes causes great harm to 

 oysters. 







SEWAGE, THE POLLUTION OF RIVERS, &c. 



It will not require many words to convince any one 

 who thinks on the subject of the direful effects of sewage 

 on oysters, for, even if they are not killed outright by the 

 withdrawal of oxygen from the water, and the substitu- 

 tion of other noxious gases, it will assuredly render the 

 oysters totally unfit for human food. The rare cases where 

 oysters have acted poisonously are to be attributed to this 

 cause. 



Although oysters have very delicate constitutions, we 

 may fairly assume that, if the transition to impure feeding 

 is very gradually reached, they will, like the human being, 

 become accustomed to larger and larger doses of the poison, 

 and will eventually become acclimatised, as it were, to 

 obnoxious aliment. 



