law, no great confidence can be placed in the returns, — those carried out of the city for burial, not having been included. From a " Report on the subject of introducing pure and wholesome water into the city of Boston, by L. Baldwin, Esq., Civil Engineer," it appears that the whole number of wells in that city in 1835, was 2,767. The water from 2,085 of these wells was drinkable, though brackish and hard, and 682 of them were bad and unfit for use. There were only seven of the city wells which yielded soft water occasionally and for washing, and from 33 of them the water was obtained by deep boring. " Within a few years," says the Report, " it has become common in Boston, and the vicinity, to bore for water, and to make what are called Artesian wells. But no certain or valuable result has grown out of these endeavors. There are 33 bored wells, only two of which are stated as furnishing soft water. The same remarks will apply to the public wells of this city, the most of which produce nothing but hard and brackish water, and none of which is sufficiently soft to authorize its use in washing clothes," &c. Lake Water is a collection of rain, spring and river water, usually more or less contaminated with putrefying organic matter. It is generally soft, and when filtered, is as good and wholesome as any other description of waters. Though lake water cannot be characterized as having any invariable qualities ; yet most of the Lakes of the United States, especially our great ones, afford a very pure water. In many of our smaller lakes the water is more or less stagnant, and of course very unhealthy. Marsh Water. This is analogous to lake water, except that it is altogether 140 stagnant and is more loaded with putrescent matter. The sulphates in sea and other waters are decomposed by putrefying vegetable matter, with the evolution of sulphuretted hydrogen ; hence the intolerable stench from marshy and swampy grounds liable to occasional inundations from the sea. Marsh water cannot be drunk with safety either by man or beast. Tests of the usual impurities in Common Water. The following are the tests by which the presence of the ordinary constitu- ents or impurities of common waters may be ascertained. 1. Ebullition. — By boiling, air and carbonic acid gas are expelled, while carbonate of lime, (which has been held in solution by the carbonic acid) is deposited. The latter constitutes the crust which lines tea-kettles and boilers. 2. Protosulphate of Iron. If a crystal of this salt be introduced into a phial filled with the water to be examined, and the phial be well corked, a yellowish-brown precipitate (sesquioxide of iron) will be deposited in a few days, if oxygen gas be contained in the water. 3. Litmus. Infusion of litmus or syrup of violets is reddened by a free acid. 4. Lime Water. This is a test for carbonic acid, with which it causes a white precipitate ( carbonate of lime) if employed before the water is boiled. 5. Chloride of Barium. A solution of this salt usually yields, with well water, a white precipitate insoluble in nitric acid. This indicates the presence of sulphuric acid (which, in common water, is combined with lime). 6. Oxalate of Ammonia. If this salt yield a white precipitate, it indicates the presence of lime, (carbonate and sulphate.) 7. Nitrate of Silver. If this occasion a precipitate insoluble in nitric acid, the presence of chlorine may be inferred. 8. Phosphate of Soda. If the lime contained in common water be removed by ebullition and oxalic acid, and to the strained and transparent water, ammo- nia and phosphate of soda be added, any magnesia present will, in the course of a few hours, be precipitated in the form of the white ammoniacal phosphate of magnesia. 9. Tincture of Galls. This is used as a test for Iron, with solutions of which it forms an inky liquor, (tannate and gallate of iron). If the test produce this effect on the water before, but not after boiling, the iron is in the state of carbonate ; if after, as well as before, in that of sulphate. Tea may be sub- stituted for galls, to which its effects and indications are similar. Fcrro cyanide of potassium yields, with solutions of the sesqui-salts of iron, a blue precipitate, 141 and with the proto-salts a white precipitate, which becomes blue by exposure to the air. 10. Hydrosulphuric Acid. (Sulphuretted Hydrogen.) This yields a dark (brown or black) precipitate, (a metallic sulphuret) with water containing iron or lead in solution. 11. Evaporation and Ignition. If the water be evaporated to dryness, and ignited in a