-1859] WRITINGS OF JOSEPH HENRY. 215 



glass tube of about 40 inches in length, closed at one end, 

 and filled with dry and warm mercury. The tube thus 

 filled was inverted with its lower end in a basin of the same 

 metal, and thus formed an arrangement similar to that of 

 an ordinary barometer, in which the pressure of the air, as 

 is well known, forces up the mercury and keeps it suspended 

 at an elevation of 30 inches, when the experiment is made 

 at the level of the sea. The space above the mercury is a 

 Torricellian vacuum j that is, a space void of all gross 

 matter, save a very attenuated vapor of mercury, which can 

 also be removed by a reduction of temperature below the 

 50th degree of Fahrenheit's scale, but the correction on this 

 account is so small that it may be neglected. Into this 

 vacuum Dr. Dalton introduced a very small quantity of 

 water, by forcing it from a small syringe into the mercury 

 at the base of the column, whence it rose to the surface and 

 was attended with an immediate depression of the mercurial 

 column, which, when the temperature of the room was at 

 60°, amounted to nearly half an inch. By this experiment 

 it was proved that water at the ordinary temperature, when 

 the pressure of the air is removed, immediately flashes into 

 steam or vapor, and that the atoms of this vapor repel each 

 other, thus producing an elastic force which depresses the 

 column of mercury. In this experiment, the quantity of 

 water introduced was but a few grains, yet it did not all flash 

 into vapor, but a portion of it remained in the form of a 

 thin stratum of liquid on the surface of the mercury. Its 

 weight, however, was insufficient to produce the observed 

 descent of the column, and its effect in this respect could 

 readily be calculated, since its weight was known. The 

 descent of the mercury was therefore due to the repulsion of 

 the atoms of vapor, and the former afforded an accurate 

 measure of the comparative amount of this force. 



The tube, as we have stated, was 40 inches long ; and since 

 the column of mercury at first occupied but 30 inches of its 

 length, the extent of the vacuum before the introduction of 

 the water was 10 inches, and afterward 10| inches. That 

 the depression of the mercury is an exact measure of the 



