WATERS AND ATMOSPHERIC DISTURBANCES. CV 



can reside in that which is of mediocre quality ; no mollusc will live in what 

 is thoroughly polluted. The phanerogamous plants thus sketch in distinct 

 traits, the characters of different streams ; but infusoria and cryptogams, and 

 particularly algas, may also enable one to judge in the matter .by the 

 modifications to which they are subject from alteration of the water. These 

 lower organisms survive after the disappearance of fish, of molluscs, and 

 of green herbs. As the alteration of the water progresses the river loses its 

 limpidity, it becomes opaline, and this gray colour resists filtration. The 

 surface is covered with froth, and the water deposits a dark, fetid slime, 

 whence bubbles of gas are liberated. Presently there appear sulphurets, 

 especially sulphuretted hydrogen, and the emanations of the river blacken 

 silver and cooking utensils that may be exposed to them. M. Gerardin 

 observed that when water contains the normal proportion of dissolved 

 oxygen it may support the life of fish and herbs. As the oxygen diminishes 

 the animals having active respiration disappear first, then those whose 

 respiration is lower. And he gives as. an example the black leech, which 

 will exist in water wherein the shrimp at once dies. 



"Waters have been directly poisoned due to the refuse from a gas tank 

 having obtained access to the river ; by mine water, chloride of lime, caustic 

 potash, the refuse from manufactories, paper mills, .bleaching grounds, 

 tanneries, or sewers; artificial root manure, sheep dipping; beer unfit for 

 consumption having been emptied into fish ponds; the overflow of peat 

 bogs, and other destructive agencies. The more rapid the current the more 

 quickly are poisons entering the stream diluted, and the less chance of their 

 being immediately destructive to the fish. Fish themselves appear to dread 

 foul water, and some of our rivers which used to afford salmon, shad, &c, are 

 no longer frequented by them. The wash occasioned by steam launches in a 

 river may destroy or cast on the banks eggs or broods of young fish, as 

 may likewise a very high tide. Water which is sufficiently pure for some 

 species, as members of the carp family and notably the gudgeon, to reside in, 

 may not be sufficiently so for those of the salmon family. Widespread 

 destruction is occasionally observed in the sea owing to some cause inimical 

 to the lives of fish, and this has been attributed to deleterious agencies from 

 the shore, poisons carried down by rivers, the eruption of some noxious 

 volcanic gases from the sea-bottom or sulphuretted hydrogen generated from 

 animal or vegetable decomposition acting on the sulphates of soda and 

 magnesia contained in the sea-water. 



2. Atmospheric disturbances and accidental causes may also be destruc- 

 tive : thus a frost or low temperature has been known to affect the sand 

 smelt, ballan wrasses, pilchards, conger, eels and other fishes ; but it is 

 remarkable how fish apparently frozen may occasionally be resuscitated. 

 Electric disturbances may be a cause of the death of fishes. Thus during 



