It may sometimes be desirable to eliminate, to some extent, the alkalinity in the process of softening water. According to Johnson1 this alkalinity should be limited to the smallest possible amount and rigid precaution should be taken to overcome it. This is done by various methods. The most common is that of saturating the water with carbon dioxid. The carbonic acid unites with the excess of alkali and forms a bicarbonate, which is quite effective in restoring the water to neutrality or even in making it slightly acid. Any process by means of which large quantities of carbon dioxid may be blown through the water is effective. This process is necessarily expensive and unless the source of carbonic acid is abundant and cheap cannot be used in a commercial way.

If these devices for an excess of caustic lime or caustic soda are not applicable, sometimes a good result may be reached by mixing the treated with an equal amount of the untreated water, which usually carries a considerable quantity of carbon dioxid in solution.