About eight weeks were spent in the City. On return to the dear country home the doctor took up his various duties and burdens, but the infirmities of age were often alluded to by him, and they no doubt delayed all of his work, which was further aggravated by a dangerous fall on his left hip and strain of the muscles of the thigh. He was extremely lame and for some time went about on crutches, which held him out of his laboratory. To him this was very trying. But he persisted. He was truly a splendid example for the younger aspirants for scientific honors. During the year he entered on a controversial article with his old friend Erasmus Darwin upon the subject of _spontaneous combustion_, and subsequently communicated to the _Medical Repository_ an account of the conversion of salt into nitre. He had positive knowledge of this fact for quite a little while, and upon the occasion of a visit by Dr. Wistar, told the latter concerning this with the request that no mention be made of it, evidently that he might have opportunity for additional confirmation. However, very unexpectedly, Dr. Mitchill published something of a similar character, therefore Priestley believing that he ought "to acquaint experimentalists in general with all that I know of the matter," announced that in 1799 when experimenting on the formation of air from water, having made use of the same salt, mixed with snow, in every experiment, always evaporating the mixture the salt was recovered dry. I collected the salt when I had done with it, and put it into a glass bottle, with a label expressing what it was, and what use had been made of it. Subsequently he treated this salt, after many applications of it, with sulphuric acid, when he remarked-- I was soon surprized to observe that _red vapours_ rose from it. An examination of another portion of the salt showed-- that when it was thrown upon hot coals ... it burned exactly like nitre. So it was a conversion of sodium chloride into sodium nitrate. That this change must have come from the _snow_ with which it had been dissolved, could not be doubted, and he further observed-- Now in the upper regions of the atmosphere ... there may be a redundancy of inflammable air ... and a proportion of dephlogisticated air. In that region there are many electrical appearances, as the _aurora borealis_, falling stars &c; in the lower parts of it thunder and lightening, and by these means the two kinds of air may be decomposed, and a highly dephlogisticated nitrous acid, as mine always was, produced. This being formed, will of course, attach itself to any _snow_ or _hail_ that may be forming ... confirming in this unexpected manner, the vulgar opinion of nitre being contained in snow. This seems to be the last communication of this character which came from the Doctor's pen.