WATEKS AND ATMOSPHERIC DISTUEBANCKS. 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 algse, 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 opce 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 j 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 safBciently 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 



