Page:History of botany (Sachs; Garnsey).djvu/514

494 and used there for its nourishment, while the part which escapes, being deprived of its phlogiston, necessarily attains a higher degree of purity.' After he had ceased his experiments with plants in 1778, he observed that there was a deposit of matter in the water in some vessels which he had used for them, and that it gave off a very 'pure air'; a number of further observations taught him that this air was given off only under the influence of sun-light; Priestley himself did not suspect that the deposit in question, afterwards known as Priestley's matter and found to consist of Algae, was a vegetable substance.

In the same year (1779) appeared the first book by, in which the subject was treated at length; it was called, 'Experiments on Vegetables, discovering their great power of purifying the common air in the sunshine and of injuring it in the shade and at night,' and was at once translated into German, Dutch and French. The title itself shows that the author had observed more and more correctly than Priestley. But he did not come to an understanding of the inner connection of the facts, till Lavoisier completed his new antiphlogistic theory. He says himself in his essay, 'On the nutrition of plants and the fruitfulness of the earth,' which appeared in 1796, and was translated into German with an introduction by A. v. Humboldt in 1798, that when he published his discoveries in 1779, the new system of chemistry was not yet fully declared, and that without its aid he had been unable to deduce the true theory from the facts; but that since the composition of water and air had been discovered, it had become much easier to explain the phenomena of vegetation. But in order to establish his priority he says on p. 56, that he had been fortunate enough to find out the real