“According to Stahl, phlogiston is a real substance, capable of being transferred from one body to another; its presence or absence making a remarkable difference in the properties of bodies, whether it add to their weight or not. Thus he concluded that oil of vitriol deprived of water, and united to phlogiston, becomes sulphur; and that the calces of metals, by the addition of the same substance, become metals.... What is now contended for is that in the oil of vitriol changing into sulphur something is lost and nothing gained, and also that a calx becomes a metal by the loss of air only. And did facts correspond to this theory it would certainly be preferable to that of Stahl, as being more simple; there being one principle less to take into our account in explaining the changes of bodies. But I do not know of any case in which phlogiston has been supposed to enter into a body, but there is room to suppose that something does enter into it....
“What has been insisted upon, as most favourable to the exclusion of phlogiston, is the revival of mercury without the addition of any other substance from the precipitate per se. In this case it is evident that mere heat ... is sufficient to revive the metal. And as what is expelled from this calx is the purest dephlogisticated air, it has been said that mercury is changed into this calx by imbibing pure air, and therefore becomes a metal again, merely in consequence of parting with that air.”
The dexterous Mr Kirwan, not long before he himself embraced the French doctrine, furnished Priestley with an argument which satisfied him that this cardinal fact can be accounted for without excluding phlogiston. “Since therefore the supposition is exceedingly convenient, if not absolutely necessary, to the explanation of many other facts in chemistry, it is at least advisable not to abandon it.”
“That calces do not become metals merely by parting with the air they contain, is evident from my experiments on heating them in contact with inflammable air, in which the inflammable air, or some necessary part of it, is undoubtedly absorbed; and though a little moisture be deposited in the process, it may well be supposed to be that which in conjunction with phlogiston constituted the inflammable air. And what can the other principle that is absorbed by the calx be but the same thing which, when united to water, is recovered again from the metal and found to be inflammable air having all the same properties with that which was employed in the revival of it. Metals therefore are not simple substances, but consist of their calces, and something else which they take from inflammable air. And as the same may also be taken from any combustible substance, it corresponds exactly to Stahl’s phlogiston, and therefore the doctrine of it is confirmed by these experiments; that is, we must still say that in all combustible substances there is a principle capable of being transferred to other substances, which when united to the calces of metals makes them to be metals, and which, united to oil of vitriol (deprived of its water) makes it to be sulphur.”
Thus was the ingenious man effectually entangled in his errors, his ingenuity helping him to deceive himself by evading the force of truth. To err is human. If Priestley saw through a glass darkly, and but dimly discerned the truth, he at least strove, so far as in him lay, to reach the light. Posterity forgives, and may well forget, his errors in grateful recognition of the many noble services he rendered to our common humanity, and in humbling recollection of the suffering and sacrifice with which those services were requited.
FOOTNOTES
[1]The Gregorian Calendar was not adopted in Great Britain until 1751. In 1752 eleven days were left out of the Calendar, September 3rd being counted the 14th. The change of style probably accounts for the confusion in the various dates of Priestley’s birth given by different writers. In Chalmers’s General Biographical Dictionary the date is given as March 18; in Allen’s American Biographical and Historical Dictionary and in Thomson’s History of the Royal Society as March 24; Corry, in his Life of Priestley, gives March 24; Hoefer, in his Histoire de la Chimie, gives March 30, probably following Dumas’s Philosophie de Chimie; Cuvier, in his Eloge, says that he was born near Bristol in 1728! In a letter to Wedgwood, dated March 23, 1783, Priestley says in a postscript “This day I complete my half century.”
[2]T. Wemyss Reid, Memoir of John Deskin Heaton, p. 7 et seq.
[3]The “Great Frost,” as it was called, which, beginning on December 26, 1739, continued with the greatest intensity till February 17, 1740. Above London Bridge the Thames was completely frozen over, and numerous booths were erected on it for selling liquor, etc., to the multitudes who daily flocked there.
[4]The Inquirer, January 16, 1904.
[5]Dr Andrew Kippis, an eminent Presbyterian, was the minister of the Prince’s Street Chapel, Westminster, and had at his disposal funds which he could employ in assisting young ministers in their education and first settlement. Priestley enjoyed his friendship through life. Kippis, who was the editor of the Biographia Britannia, was elected into the Royal Society in 1779, and served on its council.