494 Theory of the Nutrition [BOOK in. 



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 sus- 

 pect that the deposit in question, afterwards known as Priest- 

 ley's matter and found to consist of Algae, was a vegetable 

 substance. 



In the same year (1779) appeared the first book by INGEN- 

 Houss l , 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 trans- 

 lated 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 pub- 

 lished 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 dis- 

 covered, 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 



1 Jan Ingen-Honss, physician to the Emperor of Austria, practised first 

 in Breda, and afterwards in London. He was born at Breda in Holland in 

 1730, and died near London in 1799. 



