MODERN SCIENCE. 221 



pressure" That is, at a great elevation above the sea-level, say 

 at Quito (Peru), which is 9,541 ft. above the sea, and where 

 the pressure of air is only 20*75 (barometric height) instead of 

 30 inches, and therefore no longer 15 Ibs. on the square inch, 

 water will boil at 194*2 Fahr., or 72 Reaumur, or 90 Cent. 

 That is, water will boil at about 18 less than at the sea-level, 

 where it boils at 212 Fahr. the boiling point lowering i 

 Fahr. (or 0*55 Cent.) for every 590 feet of elevation. But the 

 boiling point varies in different liquids : sulphurous acid boils 

 at 10 Cent., water at 100 Cent., mercury at 320 Cent., cad- 

 mium at 860 Cent., zinc at 1,040 Cent. All these facts 

 were determined after Franklin had demonstrated the law. 

 Franklin, next, established the IDENTITY OF LIGHTNING AND 

 ELECTRICITY (1749) the lightning being a spark several 

 miles long, instead of an inch or a few inches, passing from 

 one cloud to another. Through a simple but most ingenious 

 experiment by means of a kite, cord, and key, he caused 

 electricity to pass from a cloud to the key and collect there- 

 in, thus proving lightning to be electricity and thunder to be 

 an electric phenomenon. Franklin invented the LIGHTNING 

 CONDUCTOR (1755) an application of his discovery of the 

 power of points. For, on a sphere or globe, " the electric 

 density is everywhere uniform," but the further a body is 

 removed from the spherical shape, the less regular is its 

 accumulation hence at the point of a rod the electric density 

 will be greatest. The lightning conductor has the double 

 effect of attracting the (say positive) discharge of electricity 

 from a cloud and dispersing it into the soil, and of discharging 

 into the atmosphere the (say negative) electric fluid with- 

 drawn from the soil by the influence of the cloud ; so that it 

 prevents the accumulation of electricity on the surface of the 

 earth, and also restores clouds to their normal state. Franklin 

 proposed a theory of electricity, called the single-fluid 

 hypothesis ; but the theory of Symmer, called the double-fluid 

 hypothesis, because it supposes the existence of two fluids, is 

 the one generally accepted, explaining as it does in a more 

 satisfactory manner most of the electric phenomena. To 

 Franklin, however, are due the terms "positive " and " negative? 

 used to distinguish the two kinds of currents. He called 



