‘All that I should be able to do with water would be to expel by heat all the air it contains, and then examine, by nitrous air, how much phlogiston that air contains, but it is very possible that the fitness of water for irrigating meadows may depend upon something besides the phlogiston it contains. Experiment alone can determine these things. I never heard before of the inference, you say, has been drawn from my doctrine with respect to the use of light in vegetation. I know of no use that light is of to the _soil_. The whole effect is on the _living plant_, enabling it to convert the impure air it meets with in water or in the atmosphere into pure air. When that end is effected that water is of no further use to it. Plants will not thrive unless both their leaves and roots be exposed to air in some degrees impure. This I have fully ascertained, but I am afraid that the doctrine is not capable of much practical application. ‘I know of no method of conveying phlogiston to the roots of plants but as combined with water, and this seems to be done in the best way by a mixture of putrid matter. Water will not imbibe much inflammable air. I find volatile alkali to contain much phlogiston. It is indeed almost another modification of the same thing. ‘Since my last, I have hit upon various methods of converting water into permanent air. It is sufficient to give it something more than a boiling heat. If I only put an ounce of water into a porous earthen retort, I get a hundred ounce measures of air from it, and when I have, in this manner, got near an ounce weight of air from the same retort, it has not weighed one grain less than it did. ‘I shall be glad to hear the result of your experiments, and am truly sorry that I can do so little for you. ‘J. PRIESTLEY.’ ‘Birmingham: March 31, 1783.