666 REPORT—1899. 
confirmatory of those arrived at six years before. It is true that he generally 
found evidence of an evolution of oxygen by the plants, but occasionally the air 
was less ‘ pure’ at the end of an experiment than it was at the beginning, and this 
occurred in a sufficient number of cases to lead Priestley to doubt to some extent 
the accuracy of his previous conclusions. On the whole, however, he still thinks it 
probable that the vegetation of healthy plants has a salutary effect on the air in 
which they grow. 
The reason for this want of complete consistency in these later experiments 
was, of course, his failure at that time to recognise the important influence of 
light in bringing about the evolution of oxygen, an explanation which was given 
shortly afterwards by Ingen-Housz. 
Priestley’s attention was now taken up with another observation, which led 
him within a very short distance indeed of the discovery that the evolution of 
oxygen by plants is conditioned not only by a sufficient degree of illumination, but 
also by the pre-existence of carbon dioxide. It is the more necessary to treat of 
this point somewhat in detail, since it is a part of his work which has received but 
scanty justice at the hands of recent writers, who have apparently failed to see 
how much our modern conceptions of plant nutrition really owe to the initiative 
of Priestley. In his ‘ History of Botany,’ Sachs deals very unfairly with Priestley 
in this respect, owing to a want of intimate knowledge of his writings, and to the 
lack of anything like perspective in estimating the relative merits of his contem- 
poraries Ingen-Housz and Senebier, whose position can only be completely under- 
stood after a careful study of their numerous original memoirs, some of which are 
by no means readily accessible. 
In the course of his experiments on plants partially immersed in water more or 
less fully impregnated with ‘fixed air, Priestley had observed a fact which had 
not escaped the notice of Bonnet at an earlier date, that bubbles of gas arose spon- 
taneously from the leaves and stems, and it occurred to him that an examination of 
the nature of this gas by means of his new eudiometric process ought to settle the 
question whether plants really do contribute in any way to the purification of 
ordinary air. It was in June, 1778, that he put this to the test, and he found that 
the air thus liberated was much richer in oxygen than ordinary air. On removing the 
plants he found to his astonishment that the water in which they had been placed, 
and which had a considerable amount of ‘ green matter’ adhering to the sides of 
the phials, still continued to evolve a gas which increased in amount when the 
vessels were placed in sunlight. On testing this gas with his eudiometric process, 
he found that it consisted to a great extent of ‘dephlogisticated air’ or oxygen ; 
in fact, from the experimental results which he gives it is evident that the gas 
contained from 74 to 85 per cent. of oxygen. Having observed that the ‘green 
matter’ appeared much more readily in pump water than in rain or river water, 
and knowing that pump water contained considerable amounts of ‘fixed air, he 
was led to make a series of experiments with water artificially impregnated with 
carbon dioxide, which left no doubt in his mind that the production of the ‘ green 
matter’ and the evolution of dephlogisticated air were in some way due to the 
presence of ‘fixed air’ Up to this point Priestley was following a path which 
seemed about to lead him to a complete solution of his previous difficulties. He 
had beyond all question succeeded in showing not orly that the evolution of oxy- 
gen was dependent on the pre-existence of carbon dioxide, but that light was also 
required for the process. It only wanted in fact the recognition of the vegetable 
nature of the alga which constituted his ‘ green substance’ to bring these observa- 
tions into line with his previous work, and to complete a discovery which would 
have eclipsed in importance all the others with which Priestley’s name is asso- 
ciated. It was just this one step which he most provokingly failed to take. Itis 
true that he examined the ‘ green substance’ under the microscope, but owing to 
want of skill in the use of the instrument, and also to his defective eyesight, he 
was unable to determine its true nature, and unfortunately adopted the view that 
it had merely a mechanical action in separating the oxygen from the water, and, 
to use his own words, that ‘it was only a circumstance preceding the spontaneous 
emission of the air from water.’ He was in fact now inclined to regard the 
