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168 copper and zinc). This battery, although non-existent now, was the means of producing valuable discoveries in the hands of Gay-Lussac and Thénard.

Davy proved that oxygen was not the acidifying principle of acids, as stated by Lavoisier; and he led the way to the ultimate definition of an acid.

In 1803 Davy was elected F.R.S.; in 1807, Secretary of the Royal Society; and in 1820, its President. During these years a vast number of papers were published by him; and he published the following books: Elements of Agricultural Chemistry (1813); Elements of Chemical Philosophy; Salmonia, or Days of Fly-fishing; and Consolation in Travels, or the Last Days of a Philosopher (published in 1831, two years after his death). The last-named book contains some finely written theories on ethical and moral questions, with descriptions of Italian scenery.

In his book on Agricultural Chemistry, Davy says:—

Agricultural chemistry has not yet received a regular and systematic form. It has been pursued by competent experimenters for a short time only. . . . I am sure you will receive with indulgence the first attempt made in this country to illustrate it by a series of experimental demonstrations. . . . It is evident that the study of agricultural chemistry ought to be commenced by some general inquiries into the composition and nature of material bodies, and the law of their changes. The surface of the earth, the atmosphere, and the water deposited from it, must either together, or separately, afford all the principles concerned in vegetation; and it is only by examining the chemical nature of these principles that we are capable of discovering what is the food of plants,