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TRANSACTIONS OF SECTION B. 665 
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 unquestion- 
ably the starting-point for all succeeding workers, were the result of some hap- 
hazard method of working, and of one of those happy chances to which he is in 
the habit of attributing some of his most important discoveries. However much the 
element of chance entered into some of his work, and in this respect I think Priestley 
often does himself injustice, the discovery 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, written 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, combustion, 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 philo- 
sophy, 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 obser- 
vation led 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 wholesome 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 repeated 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 in a confined and unnatural situation, 
cannot but render it highly probable that the injury which is continually done to 
the atmosphere by the respiration of such a number of animals, and the putre- 
faction of such masses of both vegetable and animal matter, is, in part at least, 
repaired by the vegetable creation ; and notwithstanding the prodigious mass of 
air that is corrupted daily by the above causes, yet if we consider the immense 
profusion of vegetables 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 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 criti- 
cism 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 determined 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 
1 Nitric oxide was discovered by Priestley in 1772, and was described by him 
ander the name of ‘ nitrous air.’ 
