CHEMICAL SCIENCE. 199 



salt, there is a disengagement of heat. When solutions of two neu- 

 tral salts are mixed, and a precipitate formed from their mutual de- 

 composition, there is always a disengagement of heat, which, though 

 small, is perfectly definite in amount. The diamond disengages 

 7,824 units of heat during its combustion in oxygen gas, in the form 

 of graphite, 7,778 units, and in that of wood charcoal, 8,080 Dr. 

 Andrews before the British Association at Birmingham. 



ACTION OF WATER ON LEADEN SERVICE-PIPES. 



THE following extracts are taken from a very important investiga- 

 tion by Prof. E. N. Horsford, of Cambridge, published in the Pro- 

 ceedings of the American Academy for 1849. The researches were 

 undertaken at the request of the Board of Consulting Physicians of 

 the city of Boston, and must be considered as conclusive in regard 

 to the long-agitated question concerning the action of lead on wa- 

 ter : " The waters used by man, in the various forms of beverage 

 and for culinary purposes, are of two classes, viz. : 1. Open waters, 

 derived from rain-falls and surface-drainage, like ponds, lakes, riv- 

 ers, and some springs ; and 2. Waters concealed from sunlight, and 

 supplied by lixiviation through soils or rock, or both, of greater or 

 less depth, such as wells and certain springs. 



" They differ (a) in temperature ; well-water, through a large part 

 of the year, is colder than lake, pond, or river water ; (b) in the per- 

 centage of gases in solution ; recently drawn well-water, in summer 

 particularly, parts with a quantity of air upon exposure to the sur- 

 face temperature. In winter these relationships must to some extent 

 be inverted, in high latitudes for a longer, and in lower latitudes for 

 a shorter period, (c) They differ in the per centage of inorganic 

 matter in solution; well-waters contain more; (d) in the relative 

 proportions of salts in solution ; well-waters contain more nitrates and 

 chlorides ; and (e) in the per centage of organic matter ; well-waters 

 contain less. 



" Relations of Lead to Air and Water. (a} Lead is not ox- 

 idated in dry air, or (b) in pure water deprived of air. (c) It is 

 oxidated in water, other things being equal, in general proportion 

 to the amount of uncombined oxygen in solution, (d) When pres- 

 ent in sufficient quantity, nitrates in neutral waters are to some ex- 

 tent reduced by lead, (e) Both nitrates and chlorides promote the 

 solution of some coats formed on lead. (/) Organic matter influ- 

 ences the action of water upon lead. If insoluble, it impairs the 

 action by facilitating the escape of air ; if soluble, by consuming the 

 oxygen in solution, and by reducing the nitrates when present. The 

 green plants, so-called, and animalcules which evolve oxygen, are 

 abundant in open waters in warm weather only, and of course when 

 the capacity of water to retain air in solution is lowest; so that al- 

 though oxygen is produced in open waters by these microscopic or- 

 ganisms, it does not increase the vigor of their action upon lead. 

 (g) Hydrated peroxide of iron (iron-rust) in water is not reduced by 



