NEGATIVE ELECTRICITY FROM HOT CALCIUM AND FROM LIME. 161 



The variations in the value of Q are very considerable nearly as large as in the 

 case of the calcium cathode. Several other sets of observations of the negative leak 

 from a lime-covered cathode were made, and in all cases there were considerable 

 variations in the values of the constant Q, calculated from the experimental numbers. 

 It should be mentioned that when the temperature was 877 C. a pale glow was observed 

 in the gas on one side of the cathode. This extended to both sides of the cathode on 

 raising the temperature to 918 C., and became brighter at each subsequent increase of 

 the temperature. The appearance of the glow was not marked by any abnormal increase 

 in the current, as will be seen by the perfect continuity of the curve in fig. 5. The 

 large values of Q obtained at this point seem to be purely accidental. From a large 

 number of experiments with lime cathodes the mean value of Q, calculated from 

 observations made just when the discharge became luminous, was not greater than 

 the average value for the whole series of observations. The appearance of the 

 luminous discharge will be treated of more fully in a later part of this paper. 



The negative leak from the lime was tested again a few hours later. It was found 

 to have decreased very considerably, and the luminous discharge did not now appear 

 until the cathode had been raised to a much higher temperature than before. This 

 was found to be a general rule, namely, that long continued heating of the lime 

 diminished the negative leak. In the present case, after heating for about two hours 

 to 1000 C., the negative leak was only of about the same magnitude as in the case 

 of the calcium cathode. This will be seen by comparing the numbers in the following 

 table with those given for calcium in Table IV. : 



TABUS VI. Negative leak from lime after heating to 1000 C. for two hours in 

 helium gas at a pressure of 3'236 millimetres. 



Temperature, Negative leak, Temperature, Negative leak, 



C. amperes per square centimetre. * C. amperes per square centimetre. 



942 6-84xlO- 1226 2-24 xlO' 8 



I"* 1 "' l-22x!0- 7 1290 1 81x10-' 



1170 l-51x!0- 1316 3-42xlO- 



The mean value of Q obtained from these observations is 1/34 x 10* much greater 

 than the mean value given in Table V. The luminous discharge was not observed 

 in these experiments until the temperature was raised to 1316C. 



The mean value of the constant A calculated from the experimental numbers 

 tabulated in Table V. is 6 -42 x 10", so that the equation for the current per square 

 centimetre from lime at the absolute temperature 6 is 



x = 6-42 x io ll e --* '/. 



The currents calculated by means of this formula are given in the third column 

 of Table V. 



From these figures it will be seen that the formula only roughly represents the 

 VOL. ccvn. A. Y 



