“Several times I divided the quantity of air in which the candle had burned out into two parts, and putting the plant into one of them left the other in the same exposure, contained also in a glass vessel immersed in water, but without any plant, and never failed to find that a candle would burn in the former but not in the latter.... This remarkable effect does not depend upon anything peculiar to _mint_, which was the plant that I always made use of till July 1772; for on the 16th of that month I found a quantity of this kind of air to be perfectly restored by sprigs of _balm_, which had grown in it from the 7th of the same month. “That this restoration of air was not owing to any aromatic effluvia of these two plants not only appeared by the _essential oil of mint_ having no sensible effect of this kind, but from the equally complete restoration of this vitiated air by the plant called _groundsel_, which is usually ranked among the weeds and has an offensive smell. Besides, the plant which I have found to be the most effectual of any that I have tried for this purpose is _spinach_, which is of quick growth, but will seldom thrive long in water.” The next paper on “Inflammable Air” is of slight importance, and indeed is full of errors. Priestley made no distinction between the inflammable air obtained by the action of acids on metals (hydrogen) and that formed by the destructive distillation of coal and other organic substances (marsh gas or carbonic oxide, or mixtures of the two), and his inability to distinguish these different gases accounts for many of the phenomena he observed and which he confesses himself unable to explain. The most sagacious observation in the memoir has reference to the colour of the electric spark in the different gases which he accurately describes. The paper on “Air Infected with Animal Respiration or Putrefection” may be considered as the complement of that on “Air in which a Candle has burned out,” and is no less valuable.