Attention has been called already to the fact that water is a great solvent. It acts with vigor upon many mineral substances in the soil and dissolves gases and other materials in the atmosphere. Water thus charged with other re-agents becomes more active on certain mineral substances. Thus the solvent power of water may be increased for some substances by dissolving other substances therein. This is notably illustrated in the case of limestone which is only slightly soluble in pure water but very soluble in water carrying carbon dioxid in solution. Such water passing through layers of limestone becomes heavily impregnated with dissolved carbonate of lime. On the water emerging from the spring or the well the carbon dioxid escapes and the excess of lime is precipitated. When such water is boiled in the tea kettle all of the carbon dioxid escapes and a part of the carbonate of lime is precipitated. In families where such water is used the tea kettle may often become incrusted with a layer of carbonate of lime. In addition to the ordinary substances found in water, those which are of more rare occurrence are frequently found. This is illustrated in the fact that some waters contain small quantities of lithium, in fact lithium seems to be a very widely distributed element and is found in many waters. As it will be seen further on, many waters claimed to be lithia waters are not so in fact, but have their lithium added after being taken from the source. There are genuine lithium waters. These waters are usually highly valued for medicinal purposes, though the value of the lithium a.

therein as a remedial agent has been very much exaggerated. Some still rarer elements, or at least those less known, are often found in water. Among these may be mentioned argon and other rare constituents of the air. Certain waters also exhibit phases of radio-activity and have thus obtained a vogue as healing agents far beyond their merits.