Distilled water, taken copiously, will soon make one sick. A number of diseases which prevail in some mountain countries are ascribed by many to drinking comparatively pure snow water. Another surprise for some, perhaps! We consider water tasteless, but _had_ it no taste we would loathe it! But we will return to this hereafter. There are two tasteless, odorless, colorless gases called respectively, “Hydrogen” and “Oxygen.” If we mix them in a vessel and apply a match, there is instantly a powerful explosion, heat is generated and there is formed—water. The two strange, invisible gases have combined and formed the well-known liquid. Whenever a substance which contains hydrogen, like wood, paper, starch, sugar, &c., burns, it forms water with oxygen of the air. In fact, these two gases are always ready, on the slightest provocation to unite. Rain, or melted snow, approaches nearest to chemically pure water, but all wells, springs, rivers and seas contain dissolved substances, in greater or less degree. Water being the most universal solvent, whenever it comes in contact with the earth, dissolves the soluble constituents of the soil through which it flows, and hence it is that on analyzing water we find solid substances in solution. Streams in their onward course take up more and more matter; rivers flow into the ocean, and in the ocean the maximum amount of solid matter in solution is found. There the rivers have been carrying their load for centuries and leaving it, since water evaporating carries nothing away with it. Besides these solid bodies some gases are dissolved by water. Many waters contain, besides mineral and gaseous bodies, organic matter—living animal and vegetable organisms or decayed substances. Having noticed the different classes of foreign ingredients in water, let us study them a little more closely, considering their effects and influences on the human system. _Mineral Ingredients._—These are the substances most frequently met with in waters, indeed, organic matter may be said to be rare in comparison to the wide distribution of the inorganic solids. An enumeration will reveal many things with which we are more or less familiar. The most common solids in well, river and sea waters, are lime, magnesia, soda, potash, iron, chlorine, sulphurous acid, silica, phosphoric acid and alumina. An analysis of an average soil will reveal the presence of all of these substances, so that it is very easy to understand how they got into the water. If we arrange some of them in another way, grouping some together, we will recognize many things common in medicine or every day life. Thus we have Chloride of sodium or common salt. Sulphate of soda or Glauber’s salt. Sulphate of magnesia or epsom salt. Sulphate of lime or plaster. Carbonate of lime or limestone. Carbonate of soda or common “soda.”