268 Prof. Thomson, A Method of Comparing the [Feb. 11, 



effect of this kind is shown by the appearance at a slightly lower 

 pressure, which however is still too high to allow the ring discharge 

 to pass, of a velvety glow over this part of the glass. This electric 

 discharge from the glass sends charged atoms into the body of the 

 bulb, and these moving under the electromotive forces endow 

 the gas with the conductivity which is evidenced by the dimming 

 of the discharge through the bulb placed in the coil B. 



Conductivity arising from Chemical Action between Two Gases. 

 When there are free charged atoms in a gas, the gas will be a 

 conductor, and its conductivity can easily be tested by the method 

 described in this paper. I thought it would be of interest to 

 test whether chemical action between gases was accompanied by a 

 temporary disengagement of free charged atoms, if this were the case 

 the gases would be conductors while the chemical action was 

 going on. The first experiments I tried consisted in letting 

 hydrochloric acid and ammonia gases mix in a bulb placed inside 

 the coil A, but though there was evidently vigorous chemical 

 combinations going on between the gases I was unable to detect 

 any sign of conductivity. A similar want of success was met 

 with when a mixture of hydrogen and chlorine at atmospheric 

 pressure was exposed to diffuse light. Here again no conduc- 

 tivity could be detected. In other cases I was more successful, 

 for when HgS and CI2 were mixed at atmospheric pressure and 

 chemical combination in consequence took place, a most decided 

 effect was produced on the bulb in B, showing that the gas in A 

 was now a conductor; the effect on B ceased after a time, but 

 if there was an excess of one of the gases to begin with it would 

 always be renewed by bringing in a fresh supply of the other 

 gas. When NO was being liberated in the bulb in A by the action 

 of nitric acid on copper and oxidised in the air, considerable 

 effect was again produced on the bulb in B, showing that this 

 oxidation was accompanied by conductivity. In another case of 

 oxidation I also observed the same effect, as I found that the 

 steam from boiling water containing phosphorus possessed con- 

 siderable conductivity. These examples are sufficient to show 

 that in many cases of chemical combination the charged atoms 

 are liberated to an extent sufficient to allow the positive atoms to 

 move one way, the negative atoms the other, under the influence 

 of the strong electromotive forces acting upon them in these 

 experiments. 



Another experiment I tried was to see whether the formation 

 of a rain-cloud was attended by any separation of positive and 

 negative electricity. A bulb containing air saturated with water- 

 vapour was placed inside A and connected with a Fleuss pump, 

 a few strokes of the pump produced a cloud in the bulb, but 



