a pointed electrode equal to a small multiple of the radius of curvature of its extremity the electric intensity falls very far below that required to produce discharge in a uniform field, so that the discharge from a pointed electrode ought to be compared with a spark whose length is comparable with the radius of curvature of the point. For such short sparks the electric intensity is very high. The electric intensity required to produce the discharge from a gas diminishes as the pressure of the gas diminishes, but not nearly so rapidly as the electric intensity for long sparks. Here again the discharge from a point is comparable with short sparks, which, as we have seen, are much less sensitive to pressure changes than longer ones. The minimum potential at which the electricity streams from the point does not depend upon the material of which the point is made; it varies, however, considerably with the nature of the gas. The following are the results of some experiments on this point. Those in the first two columns are due to Röntgen, those in the third and fourth to Precht:-- +------+-----------------------------+--------------------+ | |Discharge Potential. Point +.| Pressure 760. | | Gas. +--------------+--------------+----------+---------+ | | Pressure 205.| Pressure 110.| Point +. | Point -.| +------+--------------+--------------+----------+---------+ | | Volts. | Volts. | Volts. | Volts. | | H2 | 1296 | 1174 | 2125 | 1550 | | O2 | 2402 | 1975 | 2800 | 2350 | | CO | 2634 | 2100 | .. | .. | | CH4 | 2777 | 2317 | .. | .. | | NO | 3188 | 2543 | .. | .. | | CO2 | 3287 | 2655 | 3475 | 2100 | | N2 | .. | .. | 2600 | 2000 | | Air | .. | .. | 2750 | 2050 | +------+--------------+--------------+----------+---------+ We see from this table that in the case of the discharge from a positively electrified point the greater the molecular weight of the gas the greater the potential required for discharge. Röntgen concluded from his experiments that the discharging potential from a positive point in different gases at the same pressure varies inversely as the mean free path of the molecules of the gas. In the same gas, however, at different pressures the discharging potential does not vary so quickly with the pressure as does the mean free path. In Precht's experiments, in which different gases were used, the variations in the discharging potential are not so great as the variations in the mean free path of the gases.