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kindly put at our disposal by Sir W. Thiselton-Dyer and Dr. 
D. H. Scott. Up to the present time our experiments have not 
been carried far enough to enable us to give a positive answer 
to the main question, but they have already suggested a new 
method of attack which will enable us in the future to deter- 
mine, with a fair amount of certainty, whether any particular 
plant, growing under perfectly natural conditions, derives any 
appreciable portion of its carbon from any other source than the 
gaseous carbon dioxide of the atmosphere. 
During the course of the inquiry, many interesting side issues 
frave been raised which we believe to be of some importance 
in their bearing on the processes of plant nutrition, and it is 
to a consideration of these that I intend to devote the greater 
part of my Address. / 
I must, however, in the first place indulge in a little historical 
retrospect, and am the more tempted to do this, as far as the 
early pioneers in this branch of knowledge are concerned, since 
a critical study of their writings has shown me very clearly that 
the relative merits of some of these older workers, and the re- 
spective parts which they took in founding the true theory of 
assimilation, have in our own time been much misrepresented 
by more than one historian of science whose name carries great 
weight. 5; 
There is no chapter in the history of scientific discovery of 
greater abiding interest than that which was opened by Priestley 
in 1771, when he commenced his work on the influence of 
plants on the composition of the air around them. It has often 
been assumed that these experiments of Priestley, which were 
unquestionably the starting-point for all succeeding workers, 
were the result of some haphazard method of working, and of 
‘one of those happy chances to which he is in the habit of attri- 
‘buting some of his most important discoveries. However much 
the element of chance entered into some of his work, and in this 
vespect I think Priestley often does himself injustice, the dis- 
covery of the amelioration of vitiated air by plants was certainly 
not a case of this kind. Of all his contemporaries belonging to 
the old school of chemistry, Priestley had the clearest conception 
of the processes of animal respiration and of their identity with 
the process of combustion. This is clearly shown by his 
‘‘Observations on Respiration and the Use of the Blood,” 
which he presented to the Royal Society in 1776 This memoir, 
avritten of course from the phlogistic point of view, only requires 
translating into the language of the newer chemistry to be an 
accurate statement of the main facts of animal respiration. We 
have it on Priestley’s own authority that it was these studies 
which produced in his mind a conviction that there must be 
some provision in nature for dephlogisticating the air which was 
constantly being vitiated by the processes of respiration, com- 
bustion and putrefaction, and for rendering it once more fit for 
maintaining animal life. In his search for this compensating 
influence, which he justly regarded as one of the most important 
problems of natural philosophy, he made many attempts to 
bring back the vitiated air to its original state by agitating it 
with water, and by submitting it to the continued action of light 
and heat, and it was in the course of these systematic attempts 
that he was led to study the influence of plants in this direction. 
It was in the month of August 1771 that he made the 
memorable experiments at Leeds of immersing sprigs of mint 
in air which had been vitiated by the burning of a candle or by 
animal respiration. To quote his own words, this observation 
fed him ‘‘to conclude that plants, instead of affecting the air 
in the same manner with animal respiration, reverse the effects 
of breathing, and tend to keep the atmosphere sweet and whole- 
some when it is become noxious in consequence of animals 
either living or breathing, or dying and putrefying in it.” That 
he was fully convinced that these observations, which he re- 
peated and amplified in the following year, presented the true 
key to the problem, is sufficiently shown by another passage 
in which he says: ‘‘ These proofs of the partial restoration of 
air by plants in a state of vegetation, though ina confined and | 
annatural situation, cannot but render it highly probable that 
the injury which is continually done to the atmosphere by the 
respiration of sucha number of animals, and the putrefaction | 
of such masses of both vegetable and animal matter, is, in part 
at least, repaired by the vegetable creation ; and notwithstand- 
ing the prodigious mass of air that is corrupted daily by the 
above causes, yet if we consider the immense profusion of vege- 
tables upon the face of the earth growing in places suited to | 
their nature, and consequently at full liberty to exert all their 
powers, both inhaling and exhaling, it can hardly be thought 
NO. 1559, VOL. 60} 
but that it may be a sufficient counterbalance to it, and that the 
remedy is adequate to the evil.” 
Between the time of Priestley temporarily relinquishing his 
experiments in this direction in 1772 and his resumption of 
them in 1778, owing to the adverse criticism of Scheele and 
others, he had discovered dephlogisticated air or oxygen, and 
had elaborated his method for ascertaining the purity of air, or 
its richness in oxygen, by determining its diminution in volume 
after mixing with an excess of nitric oxide over water.' This 
method gave, of course, a much greater degree of precision to 
his results than was attainable in his earlier work, where the 
purity of the air at the end of an experiment was only deter- 
mined by ascertaining if it would support the combustion of a 
candle or allow a small animal to live in it. 
The results of his later work were published in 1779, and 
were not altogether confirmatory of those arrived at six years 
before. It is true that he generally found evidence of an evo- 
lution of oxygen by the plants, but occasionally the air was less 
“pure” at the end of an experiment than it was at the begin- 
ning, and this occurred in a sufficient number of cases to Dr. 
Priestley to doubt to some extent the accuracy of his previous 
conclusions. On the whole, however, he still thinks it p, obadble 
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 re- 
cognise the important influence of /zg/¢ 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 observ- 
ation, 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 in- 
timate knowledge of his writings, and to the lack of anything 
like perspective in estimating the relative merits of his con- 
temporaries Ingen-Housz and Senebier, whose position can 
only be completely understood 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 consider- 
able 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 thit the production of 
the ‘‘ green matter,” and the evolution of the 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 that the 
evolution of oxygen was not only dependent on the _pre- 
existence of carbon dioxide, but that light was also required 
1 Nitric oxide was discovered by Priestley in 1772, and was described by 
him under the name of ‘‘nitrous air.” 
