106 Prof. Thomson, Electrification produced hxj heating Salts. 
mica. The electrification given off by the salt was determined 
by means of a saddle-shaped conductor suspended over the salt, 
and carried by an insulated rod connected with the gold leaves of 
an electroscope, the rod was surrounded by an earth connected 
metal tube to screen it from induction. The electroscope was 
charged positively or negatively, and the rate of leak determined 
in each case. If the salt gave off positive electricity the electro- 
scope would leak faster when charged negatively than when 
charged positively, while the leak for a positive charge would be 
greater than for a negative one if the salt gave out negative 
electricity. Even when the tube was at a red heat, the tempera- 
ture most frequently employed, there was little leak of the 
electroscope with either charge if no salt were placed under the 
conductor. A large number of salts were tried, and it was found 
that the sign of the electrification given off by these when heated 
depends not on the metal but on the class of the salt. The 
results are summarized below. 
Phosphates. 
All the phosphates I have tried, viz. those of Al, Fe, Hg, Ca, 
Zn, Pb, Ag, Sn, give off positive electricity when heated, some of 
them, such as aluminium phosphate, iron phosphate, mercuric 
phosphate in very large quantities, while the phosphates of Pb, 
Ag, Sn, at a red heat give off much smaller quantities. Aluminium 
phosphate at this temperature gives off more positive electricity 
than any salt I have tried, the emission is so great that it can easily 
be measured with a galvanometer. The amount of electricity 
given off was much the same whether the salt was in air at atmo- 
spheric pressure, in air at the pressure of a few millimetres of 
mercury, in hydrogen or in carbonic acid gas. The rate of emission 
falls off with continuous heating, and after an hour or more, the 
time depends on the temperature, is only a fraction of its initial 
value. There is no visible chemical decomposition. A current of 
air was passed over the glowing salt for several hours, bubbling 
through water just after leaving the salt, at the end of the time 
however the water showed no signs of any acid reaction. This 
copious admission of positive electricity by aluminium phosphate 
facilitates the passing of the arc discharge when the anode is a 
hot wire coated with the phosphate, the arc going with distinctly 
greater ease when the anode is hot than when it is cold. The 
amount of positive electricity given off by these hot phosphates 
is of quite a different order from that given off by clean metal at 
the same temperature ; this explains why Professor Richardson, 
in his experiments on the emission of positive electricity from 
glowing wires, found that a speck of phosphorus which had acci- 
