and only two copies of the edition existed in the middle of the eighteenth century. But one of these copies was complete. It belonged to the great library of the king. It was, however, reprinted in 1777 by Gobet, who also reprinted, about the same time, the works of Bernard Palissy. After the essays, Gobet published a part of Key's correspondence with Père Mersenne relative to them, and there are probably still other letters in the manuscript correspondence of Père Mersenne. Key's book was therefore well published at the close of the eighteenth century. Nevertheless, this edition, too, is now very rare, and only three copies exist in the libraries of France.
As very few persons of this age have read the Essays of Jean Key, we have thought it might be of service and cast some light on the history of science, obscure in its beginnings, to give a brief but exact analysis of its contents. Judging by his writings, Jean Key appears to have had very extended scientific knowledge and a talent for experimentation rare in his time. He was, furthermore, in pleasant relations with most of the learned men of the period, who were accustomed to consult him in difficult cases, while he was correctly informed of all that had been done in France and other countries. Three men in particular were closely associated with his works—viz., the Sieur Brun, master apothecary of Bergerac; Deschamps, a physician of the same city; and Père Mersenne, of the order of the Minims, who had a continuous and interesting correspondence with him.
Key's experiments were undertaken at Brun's request. Brun's letter, giving as it does an idea of the confidence which Key inspired among his contemporaries and of his high reputation, merits publication entire. He wrote: "Desiring a few days ago to calcine some tin, I weighed two pounds and six ounces of the finest English metal, put it in an iron kettle on an open furnace, and, stirring it continually without making any change in the adjustment, I converted it in six hours into a very white earth. I weighed it to determine the loss, and found that it weighed two pounds and thirteen ounces. This astonished me incredibly, for I could not imagine where the surplus seven ounces had come from. I performed the same experiment with lead, and calcined six pounds. I found it had gained six ounces. I asked the cause of this from many learned men, particularly from Dr. N. (Deschamps), but none of them could explain it to me. Your brilliant mind, which soars, when it is so disposed, above the common, will find matter to occupy itself with in this. I implore you with all my affection to employ yourself with the search for the cause of so rare an effect, and to oblige me so much that by your aid I may be enlightened respecting this wonder."
Key immediately set himself to work to find the answer to this
