278 NITROGEN, 



of the same configuration in the same storm. Hail is also produced 

 by cold, but in circumstances which are entirely different. It 

 occurs only in summer or in warm climates, and when the sun 

 is above the horizon. It seems to be produced in a humid 

 ascending current of air, greatly cooled by rarefaction, which 

 has an upward velocity sufficient to sustain the falling hailstones 

 at the same place till they attain considerable magnitude. The 

 formation of hail is always attended with thunder or signs of 

 electricity ; and it has been found that small districts may be 

 protected from its devastations by the elevation of many thunder 

 rods. 



Analysis of air. A knowledge of the composition of the at- 

 mosphere followed that of its constituent gases. Various modes 

 of analysis are practised: 1. A stick of phosphorus intro- 

 duced into a known measure of air in a graduated tube, effects a 

 complete absorption of the oxygen in 24 hours. On afterwards 

 withdrawing the phosphorus the diminution of volume may be 

 observed, which always indicates 20 or 21 per cent of oxygen. 

 2. A known measure of air may be mixed with a slight excess 

 of hydrogen more than sufficient to combine with its oxygen, 

 100 volumes air, for example, with 50 volumes of hydrogen, 

 and the mixture exploded in a strong glass tube of proper con- 

 struction, by means of the electric spark. The diminution in 

 volume of the gases after combustion is observed ; and as oxygen 

 and hydrogen unite in the exact ratio of one volume of the 

 first to two volumes of the second, one-third of the diminution 

 represents the volume of oxygen in the measure of air employed. 

 The tube used for this purpose is called the voltaic eudiometer. 

 The eudiometer of Dr. Ure is an excellent instrument of this 

 kind. It is formed of a straight tube moderately stout, of about 

 FIG. 34. l-4th or 3-8ths of an inch internal diameter, 

 sealed at one end, and about 22 inches long. 

 The closed end of this tube being softened 

 by heat, two stout platinum wires are thrust 

 through the glass from opposite sides of the 

 tube, so that their extremities in the tube 

 approach within one tenth of an inch of each 

 other. These are intended for the transmission 

 of the electric spark, and are retained, as if 

 * cemented, in the apertures of the glass when 

 the latter cools. One half the tube next the 



