In a former communication to the Society, the author noticed that spring, well, and river waters, even after being boiled, usually yield, with acetate of lead, a precipitate readily soluble, in whole or great part, in acetic acid; and as the solution appeared not to be attended with effervescence, it was conceived to be due to organic matter. The author has since found that effervescence is more common than was at first supposed, and in that case the precipitate is due to the presence of carbonate of lime in the water. It was ascertained by boiling a solution of carbonate of lime in water containing excess of carbonic acid, that the trace of carbonate of lime retained in solution after ebullition, was too slight to explain the reaction of the spring waters, on the idea that it had been originally taken up by them in this way.