Even more important than hardness is purity. This may be affected by the presence of an undue amount of mineral or other matter in suspension, generally visible to the eye, and the remedy is efficient filtration.
On the other hand, the water may be contaminated by the presence of organic matter and disease germs. If this is suspected, it is best to have the water examined by an expert, whose fee would not exceed three guineas.
Generally, it may be said that water from surface wells should always be viewed with suspicion. Deep well water is safe if not contaminated in the cistern. Public company supply water is mostly free from organic matter in dangerous excess, although some notable cases of pollution have occurred and not been detected until they have caused severe epidemics.