^{page 417 note *} Since this paper was read, my attention has been directed to a passage in Dr Christison's Dispensatory, p. 155, in which he states that all pure spring-waters contain “some vegeto-animal impregnation,” the presence of which is shewn by the discoloration of the residual salts, obtained by evaporation, when farther heated. I do not know of any other chemical writer who expresses himself in equally broad terms.

^{page 418 note *} It will be found, that if the solution of acetate of lead is prepared by dissolving sugar of lead in any well or spring water, which gives a considerable cloud with that salt, and is then filtered, it is less readily affected by carbonic acid in any liquid to which it may be added, than when it has been prepared by solution in distilled water. It was a solution of the former kind that was employed in the above experiments. Of all tests for free carbonic acid in solution, the most delicate is a solution of basic acetate of lead. It instantly indicates traces of carbonic acid in distilled water, on which lime-water has no action, and barytic water a comparatively feeble one. It seems to be for carbonic acid in solution what silver salts are for muriatic acid, or barytic salts for sulphuric.