He learned to know our own delightful Franklin in one of his visits to London. Franklin was then sixty years of age, while Priestley was little more than half his age. A warm friendship immediately sprang up. It reacted powerfully upon Priestley's work as "a political thinker and as a natural philosopher." In short, Franklin "made Priestley into a man of science." This intimacy between these remarkable men should not escape American students. Recall that positively fascinating letter (1788) from Franklin to Benjamin Vaughan, in which occur these words: Remember me affectionately ... to the honest heretic Dr. Priestley. I do not call him honest by way of distinction, for I think all the heretics I have known have been virtuous men. They have the virtue of Fortitude, or they would not venture to own their heresy; and they cannot afford to be deficient in any of the other virtues, as that would give advantage to their many enemies.... Do not however mistake me. It is not to my good friend's heresy that I impute his honesty. On the contrary 'tis his honesty that has brought upon him the character of heretic. Much of Priestley's thought was given to religious matters. In Leeds he acknowledged himself a _humanitarian_, or a believer in the doctrine that Jesus Christ was in nature solely and truly a man, however highly exalted by God. His home in Leeds adjoined a "public brew house." He there amused himself with experiments on carbon dioxide (fixed air). Step by step he became strongly attracted to experimentation. His means, however, forbade the purchase of apparatus and he was obliged to devise the same and also to think out his own methods of attack. Naturally, his apparatus was simple. He loved to repeat experiments, thus insuring their accuracy. In 1772 he published his first paper on Pneumatic Chemistry. It told of the impregnation of water with carbon dioxide. It attracted attention and was translated into French. This soda-water paper won for Priestley the Copley medal (1773). While thus signally honored he continued publishing views on theology and metaphysics. These made a considerable uproar. Then came the memorable year of 1774--the birth-year of oxygen. How many chemists, with but two years in the science, have been so fortunate as to discover an element, better still probably the most important of all the elements! It was certainly a rare good fortune! It couldn't help but make him the observed among observers. This may have occasioned the hue and cry against his polemical essays on government and church to become more frequent and in some instances almost furious. It was now that he repaired to London. Here he had daily intercourse with Franklin, whose encouragement prompted him to go bravely forward in his adopted course.