When a potable water is evaporated to dryness, it leaves a residue. All of the materials forming this residue are grouped together under the term "total solids." It is an important question to know just how much of this material may be present in a water without impairing its value for potable purposes. The quantities of total solids in drinking waters vary greatly. Some waters are almost as pure as distilled water, containing only mere traces of total solids. Others contain considerable quantities of mineral salts in solution. Sanitarians differ in regard to the total quantity of these bodies which a water may contain without being condemned for potable purposes. As little as 60 parts per million in some localities would be regarded as excessive; while in many other localities, where waters are highly impregnated with mineral salts, as, for instance, gypsum waters, as much as 1,200 parts of solids per million may be tolerated.