210 Dr. H. A. Wilson on the Laws of 



two galvanometer deflexions was taken as corresponding to 

 1066° C, and the temperature corresponding to any other 

 ■deflexion was first calculated on the assumption that the 

 deflexion was proportional to the difference between the 

 temperatures of the two junctions. The u platinum tempe- 

 ratures " thus obtained have been corrected to the centigrade 

 scale by means of the table of corrections given by Oallendar 

 (Phil. Mag. Dec. 1899, p. 534). 



This method of getting the temperature was quite suf- 

 ficiently accurate for the purposes of the present investiga- 

 tion, for which it was useless to aim at a greater accuracy 

 than 5 or 10 degrees, and according to Oallendar the cor- 

 rections are much more accurate than this near 1000°, while 

 even at 300° the error is not more than 10°. 



The Pt and Pt/Rh wires dipped into mercury cups kept 

 in a water-bath at a known temperature from which copper 

 wires led to the galvanometer. 



The gas supplied to the furnace was kept at a constant 

 pressure by means of a gasometer, and the air and oxygen 

 supply-tube was provided with a w^ater manometer, by means 

 of which the pressure of the supply could be maintained con- 

 stant, if necessary, for any length of time. In this way the 

 tube could be maintained constantly at any desired tempera- 

 ture within 5° C. without difficulty. 



The current through the air and salt between the electrode 

 EE and the tube IT' due to various potential-differences 

 between them was measured by means of an Ayrton-Mather 

 galvanometer G x . The P.D. was obtained from a battery of 

 small accumulators B, and was measured by means of a 

 Braun's electrostatic voltmeter reading from 50 to 1500 volts. 

 Two commutators served to reverse the current through the 

 galvanometer only or through the whole apparatus. 



Fig. 2 shows the way in which the current with a constant 

 E.M.F. (840 volts) varies with the temperature when solutions 

 of one gram in a litre are sprayed. 



It will be seen that in each case the current at first rises 

 rapidly to a nearly constant value which in the case of KI is 

 maintained over a wide range of temperature. Near 1200° 

 the current again begins to rise rapidly, and then somewhere 

 above 1300° suddenly attains a nearly constant value. It is 

 this nearly constant value with which the present paper is 

 mainly concerned. It appears to be the maximum current 

 which the amount of salt passing through the tube can carry, 

 for it is very little affected by increasing either the tempe- 

 rature or E.M.F. 



Of course it is possible that with higher temperatures and 



