28 Prof. R. Threlfall on the Electrical 



than one had any right to expect. The temperature was kept 

 constant for a quarter of an hour after the observation, in the 

 hope of detecting any further change, should it take place; but 

 the readings remained constant. I am therefore inclined to the 

 belief that the substance dissociates on heating, and is all de- 

 composed at a temperature of about 190° C. On June 23rd, 

 when all parts of the apparatus were again at a uniform 

 temperature, the original effect had almost returned, there now 

 being a difference of pressure of 3*2 millim. between the tube 

 and the sealed space; or, in other words, partial recombination 

 of the nitrogen appeared to have taken place. 



Other matters required my attention at this time, and I did 

 not get an opportunity to continue the study of the tube till 

 August 13. On this day the pressure-difference was 3* 7 millim., 

 apparently showing that slow recombination had been going 

 on. On August 13, 14, 15, and 17 the tube was heated each 

 day up to about 190°, and curves drawn showing the relation 

 between the pressure and temperature. These curves did not 

 show a very good agreement, from the uncertainty as to cor- 

 rections above mentioned ; and I therefore will only state that 

 the effect of the heating on the 13th was to reduce the pressure 

 difference to 2 millim., on the 14th to 1*5 millim., on the 15th 

 to *6. On each day the heating took place during five hours 

 or thereabouts, and the tube was maintained at its highest 

 temperature for about an hour. On August 17th the tube was 

 exposed to a temperature of 200° C. for six hours, and then 

 kept at 110° all night; and then, on August 18th, again 

 heated to 200° C. for five hours. When the tube was cold, on 

 August 19th, the effect had entirely disappeared, the mercury 

 standing at the same height in both limbs of the tube. On 

 August 21st recombination appeared to have taken place to 

 the extent of *5 millim., and then this disappeared again be- 

 tween August 21st and September 3rd. No further change 

 having taken place by September 20th, the tube was finally 

 disconnected, and the following conclusions were provisionally 

 noted from a consideration of its behaviour. 



1. A compound is formed when the electric discharge is 

 passed over a mixture of mercury and nitrogen at a pressure 

 of about 8 millim. The action commencing slowly, and in- 

 creasing in velocity till it reaches a maximum, then decreasing, 

 but continuing slowly till at least seven-eighths of the nitrogen 

 is absorbed, the mercury being in excess. 



2. The compound formed is brownish at first, but as time 

 goes on it becomes black, probably owing to a continual 

 increase in the quantity of mercury deposited with it. 



