156 SEC. 5. MOLECULAR PHYSICS. 



Two Tubes of Torricelli, one of which ends in a sphere. 

 They served him for his experiments on atmospherical pressure 

 in 1644. The Accademia del Cimento. 



Galileo, by condensing air, had demonstrated its weight, and in his 

 dialogue on the resistance of solids, he says of water, that in suction pumps, 

 it does not rise higher than about 18 braccia, leaving the space above empty. 

 Torricelli, pondering over this fact, was led to think of what would happen, 

 if, in the place of water, mercury, which is so much heavier, were used ; for 

 he argued that by its means there would be much greater ease in obtaining a 

 vacuum, in a much shorter space than that necessary for water. He then 

 made a long glass tube of the length of about two braccia, which terminated 

 at one end in a' ball, likewise of glass, and remained open at the other; 

 through this aperture he proposed to fill the tube and the ball completely 

 with mercury ; and then holding it with his finger, and turning it upside- 

 down, submerge the orifice of the tube below the level of more mercury in a 

 large vessel ; and that being done, take away his finger and open the tube, 

 thinking that the quicksilver would detach itself from the ball, and having 

 glided down and remained suspended according to the various calculations, at 

 about the height of 1 ^ braccia, would in all probability leave a vacuum in the 

 ball above and in part of the tube. He communicated this thought to his 

 great friend Viviani, who, most anxious to see the result, agreed to the 

 experiment, which he himself carried out, and was hence the first, about a 

 year after Galileo's death, to see Torricelli's ingenious idea confirmed by the 

 fact. He hastened to his friend, who, most joyful at the news of this 

 evidence, was all the more persuaded that the weight of the air was really 

 that which was in equilibrium with the column of water or mercury. Indeed, 

 being asked by Viviani what would have happened if the experiment had 

 been made in closed space, Torricelli, after having reflected for a short time, 

 answered, the same thing ; since the air is already compressed in it. This 

 most important discovejy was communicated by the author himself to Ricci 

 in Rome, and by Ricci to Sig. de Verdus, who, in his turn, made Padre 

 Merseune acquainted with it, from whom Pascal learnt it, and made it famous, 

 as everyone knows, in his celebrated Puy-de-D6me experiment. Torricelli 

 himself in a letter to Ricci, observes ; " that it would be possible, by means 

 ' of his instrument, to ascertain when the air was lighter or heavier ; and 

 that it might be the case that air, which is most heavy upon the surface 

 ' of the earth, becomes more and more light and pure as we rise higher and 

 ' higher to the tops of the loftiest mountains." And Carlo Beriguardi in his 

 ' Circolo Pisano," published in 1643, says; "that the tube of quicksilver 

 " leaves more space empty, when placed at the top of a tower or of a 

 " mountain, than at the foot." 



STANDARDS OF THE HYDROMETERS AND THERMOMETERS USED 

 BY GOVERNMENT OFFICERS IN HOLLAND. 



Exhibited by Dr. J. W. Gunning, Professor of Chemistry at the 

 " Athenaeum illustre" Amsterdam. 



579. General Hydrometer (No. 1), with open stem and 

 variable weight. Every degree has a bulk equal to -^ of the part 

 of the hydrometer below zero. When this instrument, having the 

 arbitrary weight = W grammes, marks a degrees in a liquid, the 



