GALILEO GALILEI 29 



thus calculated he concludes, knowing the specific gravity 

 of copper to be 9, that the highest column of copper which 

 could be supported by the power of the vacuum would have 

 a height of only 18^9, that is, 2 ells. This brought him al- 

 most to the point of Toricelli's later experiment with the 

 mercury column. He also measured for the first time, cor- 

 rectly as far as the order of magnitude, the specific gravity 

 of air, for which purpose, since he did not possess an air- 

 pump, he devised special methods.^ 



Inasmuch as he then found the tensile strength of marble, 

 for example, to be four times as great as the power of the 

 vacuum alone, he was obliged to assume the existence of a 

 peculiar force apart from the vacuum, this force being differ- 

 ent in the case of different bodies, and being the internal 

 cause of their strength. The nature of this remained unde- 

 termined; the limits of knowledge at that time were reached 

 even for Galileo. For a moment he allows his Sagredo 

 (who is himself) to say in the Discorsi that he 'cannot under- 

 stand how the coherence of the smallest particles, down to 

 the very smallest of the same matter' can bring about the 

 tensile strength; the attempt is then made, 'not as absolute 

 truth, but as a still undigested idea,' to manage with the 

 power of the vacuum alone, perhaps by some roundabout 

 means. This shows how little reason was given by the state 

 of knowledge at that time to assume the existence of peculiar 

 attractive forces acting between portions of matter. Such a 

 conception needed Newton's discovery of universal gravita- 

 tion, almost one hundred years later. Galileo's discovery 

 that the strength of all bodies is overcome by gravity, as soon 

 as their dimensions become sufficiently great, is also very 

 noteworthy; bridges, houses, trees, and animals, when con- 

 structed of the same materials and in the same way, therefore 



^ What Aristotle put forward as the weighing of air, namely com- 

 parison of the weight of a bladder blown up and collapsed, can only be 

 taken, on account of the buoyancy of the air, as the apparent confirma- 

 tion of a previously formed opinion, as is often the case with Aristotle. 



