Soon after this, a question, involving very important consequences in physics, agitated the scientific world, and the position of the two Pascals was such, that their attention could not fail to be drawn to the consideration of it. The mechanical properties of the atmosphere had previously been inquired into by Galileo, who recognised in it the quality of weight. This philosopher, however, notwithstanding the wonderful sagacity which his numerous physical discoveries evince, failed to perceive that the weight of the atmosphere, combined with its fluidity and elasticity, opposed a definite force to any agent by which the removal of the atmosphere from any space was attempted. This resistance to the production of a vacuum had long been recognised, and was in fact expressed, but not accounted for, by the phrase, "nature's abhorrence of a vacuum." Whatever meaning he may have attached to it, Galileo retained this phrase, but limited its application, in order to embrace the phenomenon, then well known, that suction-pumps would not raise water more than about thirty-five feet high; and although "nature's abhorrence of a vacuum" raises the water thirty-five feet, to fill the space deserted by the air, which had been drawn out by the piston, yet above that height a vacuum still remained; which fact Galileo expressed by saying, that "thirty-five feet was the limit of nature's abhorrence of a vacuum." 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.