That Galileo should have missed a discovery as important as it was obvious, is the more remarkable from the circumstance of its having been actually suggested to him by one of his own pupils. A letter from Baliani to Galileo is extant, dated in 1630, in which the writer says that Galileo, in one of his letters to him, having taught him that air has sensible weight, and shown him how that weight might be measured, he argued from thence that the force necessary to produce a vacuum, was nothing more than the force necessary to remove the weight of the mass of atmosphere which presses round every object, just as water would press on any thing at the bottom of the sea.[54]
Torricelli, the pupil of Galileo, next took up the problem. He argued, that if the weight of the atmosphere were the direct agent by which the column of water is sustained in a pump, the same agent must needs exert the same amount of force in sustaining a column of any other liquid; and, therefore, that if a heavier liquid were used, the column sustained would be less in height exactly in the same proportion as the weight of the liquid forming the column was greater. Mercury, the heaviest known liquid, appeared the fittest for this purpose. The experiment was eminently successful. The weight, bulk for bulk, of mercury was fourteen times greater than that of water; and it was found that, instead of a column of thirty-five feet being supported, the column was only thirty inches, the latter being exactly the fourteenth part of thirty-five feet.
Various ways of further testing the evident inferences to be drawn from this beautiful experiment, were so obvious, that it is impossible to suppose the illustrious philosopher to whom we are indebted for it, would not have pursued the inquiry further, had not death, almost immediately after this, prematurely removed him. The experiment became known, and excited much interest in every part of Europe; and Mersenne, who had an extensive scientific correspondence, having received an account of Torricelli's investigation, communicated the particulars to Pascal. Always reluctant to surrender long established maxims, the philosophers of that day rejected the solution of the problem given by Torricelli, and still clung to the maxim that "nature abhors a vacuum." The sagacity of Pascal, however, could not be so enslaved by received notions; and he accordingly, assisted by M. Petit, applied himself to the discovery of some experimental test, of a nature so unanswerable as to set the question at rest. The result was the celebrated experiment on the Puy de Dôme, the first and most beautiful example of an "experimentum crucis" recorded in the history of physics.