Of acetate of soda a 100th part of the water is a prese: Of arseniate of soda 12,000th " Of phosphate of soda 30,000th " Of hydriodate of potash 30,000th " Of muriate of soda 2,000th Of sulphate of lime 4,000th " Of nitrate of potash 100th rvative. 144 The sulphates of soda, magnesia, lime, and the triple sulphate of alumina and potash, possess about the same preservative power ; which appears to depend on the acid, not on the base of the salt. The general results of Dr. Christison's inves- tigations, appear to be, that neutral salts in various, and for the most part minute , proportions, retard or prevent the corrosive action of water on lead — allowing the carbonate to deposite itself slowly, and to adhere with such firmness to the lead as not to be afterwards removed by moderate agitation, — adding subsequently to this crust other insoluble salts of lead, the acids of which are derived from the neutral salts in solution, — and thus at length forming a permanent and impermeable screen in the form of a film over its surface, through which the action of the water cannot any longer be carried on. These films are composed of the carbonate of lead, with a little of the muriate, sulphate, arseniate, or phosphate of lead, according to the nature of the acid in the alkaline salt, which is dissolved in the water. The follow- ing general conclusions may therefore be considered as sufficiently established. 1. Lead pipes ought not to be used for the purpose of conducting water, at least where the distance is considerable, without a careful examination of the water to be transmitted. 2. The risk of a dangerous impregnation with lead is greatest in the instance o f the purest waters. 3. Water, which tarnishes polished lead when left at rest upon it in a glass vessel for a few hours, cannot safely be transmitted through lead-pipes without certain precautions ; and conversely, it is probable, that if lead remain untarnished, or nearly so, for 24 hours in a glass of water, the water may be safely conducted through lead-pipes. 4. Water which contains less than about an 8000th of salts in solution, can not be safely conducted in lead pipes without certain precautions. 5. Even this proportion will prove insufficient to prevent corrosion, unless a considerable part of the saline matter consists of carbonates and sulphates, es- pecially the former. 6. So large a proportion as a 4000th part, probably even a considerably larger proportion, will be insufficient, if. the salts in solution be in a great measure muriates. 7. In all cases careful examination should be made of the water after it has been running a few days through the pipes; for it is not improbable that other circumstances, besides those hitherto ascertained, may regulate the preventive influence of the neutral salts. 8. Where the water is of sufficient purity to act on lead, a remedy may b e found, either, in leaving the pipes full of water and at rest for three or four months , 145 or by solution of phosphate of soda ; in the proportion of about a 25,600th part.* Dr. Kane, however, seems to differ from Dr. Christison in opinion on this subject ; for after having mentioned the crust which gradually forms on the interior of the cistern, and assists in protecting it from the oxidizing action of the air, he remarks, " no danger is therefore to be apprehended from the supply of water to a city being conveyed through leaden pipes, and preserved in leaden cisterns ; for all water of mineral origin dissolves, in filtering through the layers of rocks in its passage to the surface, a sufficiency of saline matters to serve for its protection." Now, to apply these results to the water of the Croton ; as this holds in solution only about one 18,000th part of salts, it must, according to Christison, exert a corroding influence on the lead-pipes. Dr. Dana, of Lowell, has lately investigated this subject and detected lead in the water which had passed through the leaden- pipes for the distribution of water in the city of Lowell. The first examination was made from a sample of water taken from the source or spring-head before it had entered the leaden pipes, when the specific gravity was found to be 1,000,18. The pint, on evaporation to dryness, yielded 2.37 grains of solid matter. The solid contents of an imperial pint were found to be, Grains. Chloride of Sodium, - 1.54 Chloride of Magnesia, - - - 0.71 Sulphate of Lime, - - - 0.128 A trace of Carbonic acid, Grains, 2.378 Excess in the course of analysis - .008 The second examination was made of