[76] On the same day he decomposed Soda with somewhat different phenomena.
[77] He alludes to a Paper, entitled "On the Quantity of Carbon in Carbonic Acid, and on the Nature of the Diamond; by William Allen, Esq. F.R.S. and William Hasledine Pepys, Esq." Communicated by Humphry Davy, Sec. R.S. M.R.I.A. Read June 16, 1807.
[78] He here alludes to a train of research, which will be considered hereafter.
[79] This observation relates to the magnificent battery constructed by Mr. Children, of which he presented an account to the Royal Society, in a Paper read in November 1808, entitled, "An Account of some Experiments, performed with a view to ascertain the most advantageous Method of constructing a Voltaic Apparatus, for the purpose of Chemical Research. By John George Children, Esq. F.R.S." The great battery described in this Paper consisted of twenty pairs of plates, each plate being four feet high by two feet wide: the sum of all the surfaces was ninety-two thousand one hundred and sixty square inches, and the quantity of liquid necessary for charging it, one hundred and twenty gallons. At the same time he constructed another battery, which consisted of two hundred pairs of plates, each being only two inches square. In the one case, then, he commanded extent of surface, in the other, extent of number; and by a series of comparative experiments, he fully established the theory of Davy (page [246]), that the intensity of Electricity increases with the number, and the quantity with the surface.
[80] The Pyrophorus of Homberg, of which a description is to be found in the Mémoires de l'Academie, for 1711, was made by mixing together any combustible body, as gum, flour, sugar, charcoal, &c. and alum, and then, after roasting the mixture till it was reduced to a dry powder, exposing it in a matrass to a red heat. In this process, the theory of which was first explained by Davy, the potash of the alum is converted into potassium, which, by its absorption of oxygen from the atmosphere, generates heat, and sets fire to the charcoal contained in the powder.
[81] The 'Potassa Fusa' of our Pharmacopœia generally contains a small proportion of the peroxide, and will therefore effervesce when thrown into water.
[82] I received the above jeu d'esprit from the late learned Orientalist, Mr. Stephen Weston, only four days before his death. Since its publication in the first edition of this work, a chemical friend sent me the following improved version of it.
Says Davy to Baryt, "I feel strong temptation
To effect by my art your deoxidation;
And the money I've got in my pocket I'll bet all,