NEGATIVE ELECTRICITY FROM WOT CALCIUM AND FROM LIME. 159 



per square centimetre, given in Table IV. is 171 x 10 4 , so that the equation for the 

 current per square centimetre from calcium at (al**olute) becomes 



The values of the negative leaks calculated from this formula are placed alongside 

 the observed values in Table IV. 



The agreement between the observed and calculated currents is not nearly so good 

 as in the case of platinum. This is no doubt due to the difficulties attending the use 

 of calcium. In the course of the experiments a note was made in the laboratory 

 book that the currents recorded at 1050 C., 1065 C., and 1310 C. were steadily 

 increasing during the observations. We see from the table above that the observed 

 currents at these temperatures were considerably too low. If it had been practicable 

 to have left the apparatus for a few minutes before measuring the leak, the values 

 of the currents at these temperatures would, no doubt, have been nearer to the 

 theoretical value. The experiments, however, show in a quite satisfactory manner 

 that at a given temperature the rate of emission of negative corpuscles from calcium 

 is much greater than in the case of platinum ; and by applying the results to the 

 formula employed we see that this increased rate of emission is due to a decrease in 

 the value of the constant Q, that is, to a decrease in the energy required to set free 

 the corpuscles from the surface of the platinum ; for on any theory of the negative 

 leak Q is a measure of the work required to produce a gramme molecular weight of 

 corpuscles. The value found for Q for calcium (7 '29 x 10 4 ) lies between the value 

 found for platinum (T219 x 10 8 ) and that obtained by RICHARDSON for sodium 

 (6 '32 x 10 4 ). This is what one would expect, and indicates that the amount of 

 energy required to liberate the corpuscles is less the more electropositive the metal. 



(4) The Negative Leak from Lime. 



When the series of observations recorded in Table IV. had been made, the calcium 

 on the platinum strip was oxidised to lime by letting into the apparatus some pure 

 dry oxygen. The oxygen was prepared by the electrolysis of water and then passed 

 over sticks of caustic potash and some fused calcium chloride. It was let into the 

 apparatus to a pressure of 3 or 4 millims. On gradually raising the temperature of 

 the cathode the negative leak was at first only slightly greater than before the 

 oxygen was admitted, but soon it increased very rapidly as the calcium oxidised, and 

 a pale glow appeared in the discharge tube. After this luminous discharge had once 

 appeared the negative leak at all temperatures was much greater than before the 

 oxygen had been admitted into the apparatus. The following table gives the 

 negative leaks under a potential difference of 40 volts at various temperatures in 

 helium, the excess of oxygen having been absorbed by sending a discharge for some 



