
LOMNPUrIONS OF LIGHT, ETC. 

substances. So much is this the case that chemists distinguish substances 
as soluble or insoluble in water. Some which are not affected at ordinary 
temperatures dissolve in the superheated water of deep-lying strata, as is 
noticeable in geyser deposits; while other substances held in suspension 
are concentrated by evaporation and rendered insoluble by heat and pre- 
cipitated; for instance the deposits in a teakettle or steam boiler. 
When there is much vegetal or animal matter present, or when 
the hardness is due to salts of magnesia or to sulphate of lime, the water 
is not well suited for drinking purposes; and pollutions by the waste 
materials of factories and dwellings, by sewage or similar contaminations, 
are the cause of certain specific diseases which become epidemic; and inju- 
rious micro-organisms belonging to the class of fungi, alga, bacteria and 
the large class of Schizomycetes, when present in drinking water, are the 
direct cause of typhoid and typhus fevers, diphtheria, dysentery and numer- 
ous other stomach and bowel complaints. To destroy these, thorough 
aeration is better than filtration, as most of the disease organisms are so small 
that mechanical filters will not remove them and their spores. Boiling is 
the best sterilization. 
Any good drinking water is all that is required for the aquarium, 
though soft water is natural to the Carp family. Gradual changes in tem- 
perature also do not affect fishes if not too long sustained or excessive; but 
for those which naturally inhabit cold water streams, attention must not 
only be given to the character of the water but also to the required tem- 
perature, and what may be either excessively warm or cold avoided. 
A constant danger in the aquarium is the fouling of the water by 
the decomposition of dead animals. ‘This is indicated by a local cloudi- 
ness, a greasy surface, a foul odor, and later by the behavior of the fishes, 
and is followed by the death of all the inmates and the decay of the plants 
if the cause is not removed. ‘This is more often due to dead scavengers 
than fishes. 
Brook and river water abundantly contain the spores of alge, which 
are beneficial in clearing the water and furnish food for the inmates, but 
if these are excessive or objectionable, the water should be filtered or spring 
water used which has been left standing to acquire oxygen and so assume 
the character of stillwater. 
In a healthy aquarium the water must be clear and colorless though 
a faintly yellow or green tint is not objectionable, the one due to the 
decomposition of chlorophyll, the other to the presence of alge. Com- 
parison of condition is best made by taking a glassful and comparing it 
with a similar glassful of the water of a well-conditioned aquarium or with 
filtered water. 
