If Priestley was not himself a poet, he was at least the cause of poetry in another. Miss Aikin once told him that it was the perusal of some verses of his that first induced her fledgling muse to soar—so that, he adds, “this country is in some measure indebted to me for one of the best poets it can boast of.” No example of Priestley’s abilities as a “versifier” has come down to us, but in that dainty little sketch of the Warrington society, by Miss Lucy Aikin, from which we have already quoted, allusion is made to his accomplishment.
“Both bouts rimés and vers de société were in fashion with the set. Once it was their custom to slip anonymous pieces into Mrs Priestley’s work-bag. One ‘copy of verses,’ a very eloquent one, puzzled all guessers a long time; at length it was traced to Dr Priestley’s self.”
To the man of science the special interest of Priestley’s connection with Leeds arises from the fact that he began there that fruitful series of inquiries, relating to what he called “the doctrine of air,” which eventually raised him to the position of one of the greatest chemical discoverers of his time. The house in which he first lived whilst at Leeds was in Meadow Lane and adjoined the public brew house of Jakes and Nell. He was thereby led, in the outset, to amuse himself by making experiments on the “fixed air,” or carbonic acid, which is largely produced in the process of fermentation. When he removed to his second house in Basinghall Street, on the site where the schools now stand, he was under the necessity of making the fixed air for himself; and, as he distinctly and faithfully notes in his various publications on the subject, he was led to make one experiment after another until he became, what he does not state, the greatest master of pneumatic chemistry of his age.
When he began these experiments he tells us he knew very little of chemistry. Indeed, he says he had in a manner no idea on the subject before his attention was drawn to it in a course of lectures delivered in the Warrington Academy by Dr Turner of Liverpool. But, as he says, on the whole, this circumstance was no disadvantage to him, as in the situation in which he found himself he was led to devise an apparatus and processes of his own adapted to his peculiar views. If he had been previously accustomed to the usual chemical processes he might not have so easily thought of any other; and without new modes of operation he thinks he should hardly have discovered anything materially new. His means did not permit him to purchase expensive apparatus. Indeed, this very circumstance materially contributed to his success by making his apparatus so simple that his experiments could be readily repeated and their accuracy thereby ensured.
His first contribution to Pneumatic Chemistry was published in 1772. It was a small pamphlet on a method of impregnating water with fixed air, which, being immediately translated into French, excited a great degree of attention to the subject, and this was much increased by the publication of his first experimental paper in the Philosophical Transactions of the Royal Society.
Priestley’s earliest method of impregnating water with carbonic acid consisted in exposing it to the gas above the surface of fermenting wort. This process was no doubt accompanied with many disadvantages and the resulting solution could not have been very palatable. Later on he adopted the method originally employed by Lane in 1709, although apparently in ignorance of Lane’s paper in the Philosophical Transactions, of making the gas from chalk and sulphuric acid and leading it directly into the water by means of a flexible tube provided with an intercepting bladder to retain any solid or acid substance projected from the effervescent materials in the generating flask. At about this period increased attention was being paid to the question of the supply of drinking water in the Navy, owing to the publication of Irving’s plan of making fresh water from sea-water by distillation, and Priestley conceived the idea that if some ready means could be devised of impregnating water with carbonic acid on shipboard the solution might be useful as a preventive of sea scurvy.
Priestley brought his idea to the knowledge of the Duke of Northumberland, and showed a sample of the impregnated water to Sir George Savile, who introduced him to Lord Sandwich, at that time First Lord of the Admiralty in Lord North’s Administration. The Board of Admiralty thought the matter was of sufficient importance to ask for a report from the College of Physicians, and Priestley was requested to appear before that body in order to explain and illustrate his process. The report from the College was favourable, and in consequence two war-ships were fitted with the apparatus.
The idea that scurvy, in common with other so-called putrid diseases, was due to an insufficient supply of “fixed air” in the animal economy, and that it might be cured by the administration of that gas, originated with Dr Macbride about the middle of the eighteenth century, shortly after Black had established the individuality of the gas, and it was current doctrine with the faculty at the time of Priestley’s experiments. The reasons which Macbride gave in support of his hypothesis are contained in his Essays on Medical and Philosophical Subjects, and are sufficiently ingenious to be worth stating as characteristic of much of the therapeutics of the time. Macbride assumed that substances held together, and acquired the quality of firmness, by virtue of containing a “cementing principle,” which ensured the perfect cohesion of their constituent particles, and that as putrefaction resulted in the decomposition and disintegration of substances, putridity was connected with the loss or disappearance of this cementing or cohering principle. He found that “fixed air” was invariably produced when animal and vegetable substances putrefy, that a greater amount of fixed air is produced from vegetable substances than from animal substances, and that animal and vegetable matters putrefy more rapidly when mixed than when separate, and yield more fixed air in conjunction than apart.
On the basis of these observations Macbride proceeded to explain the well-established fact that a diet mainly composed of animal food is apt to produce sea scurvy, the remedy for which is a sufficient supply of fresh vegetables, by assuming that the virtue of the vegetables was due to the evolution of a greater amount of carbonic acid in the process of digestion, the fixed air so liberated in the body counteracting, by its antiseptic powers, putridity in the circulating fluids.
We are not here concerned with the subsequent history of so-called ærated or soda-water, as it came to be called, but it is worth noting that Priestley’s account of his process contains one remark which is not without significance in view of latter-day developments. He says:—