Torricelli performed his celebrated experiment on argento vivo [quicksilver, i.e., mercury], in Florence in spring 1644. Torricelli filled a glass tube, open at one end, with mercury. Then, closing off the open end with a finger, he tipped the tube upside down and lowered it into a basin containing more mercury. He observed that the column of mercury descended only partially, stopping at a height of around 76 cm. Torricelli was convinced that the space created by the descent of the mercury in the tube was empty, and that the force holding up the column consisted of the pressure exerted by the air on the mercury in the basin. In a letter to Michelangelo Ricci of June 11, 1644, Torricelli declared that his experiment proved two fundamental concepts: first, that Nature did not abhor the void; second, that the air had weight. The results of the mercury experiment opened a period of revolutionary change, forcing a re-examination of doctrines accepted for centuries.