-1859] WHITINGS OF JOSEPH HENRY. 223 



is partially expelled. It is then suffered to cool, when by 

 the pressure of the atmosphere 

 a quantity of water is forced up 

 into the bulb. This is made to 

 boil rapidly so as to expel along 

 with the escaping steam all the 

 air. The capillary end of the 

 bulb being again plunged be- 

 low the surface of the water, 

 and the lamp withdrawn, the ^^^- ^■ 



pressure of the atmosphere will now entirely fill the bulb 

 with the liquid. The point of the capillary tube is then 

 closed by melting it in the flame of the blow-pipe, and 

 the bulb thus filled with water is again weighed. If from 

 this last weight we subtract the weight of the glass we 

 shall have the weight of the contained water. This bulb 

 with its known amount of water is next placed in the 

 glass globe a, (Fig. 3,) the long tube screwed in its place, and 

 the whole apparatus filled with dry mercury and inverted 

 in a basin i of the same metal. The mercury of course by 

 its weight will descend from the glass globe into the tube, 

 and sink until it becomes in equilibrium with the weight of 

 the atmosphere, which as we have said before, will be about 

 the height of 30 inches. The inside of the globe will then 

 be a Torricellian vacuum, and the water if released from the 

 small bulb in which it is contained would immediately flash 

 into vapor by the unbalanced repulsion of its atoms ; and 

 we can readily release them from their confinement by 

 directing upon the bulb for an instant a beam of heat from 

 the sun by a burning glass. By this means the bulb will 

 be broken, (particularly if formed of dark glass,) the water 

 will be set free, and will be converted in part at least into 

 vapor. The whole apparatus is then heated by plunging it 

 into a water bath of which the temperature is gradually 

 raised, or by heating the room in which the experiment is 

 made, until all the water is converted into vapor. By care- 

 fully noting the temperatiire at which the liquid disappears, 

 we have from the previous table the tension of the vapor at 



