458 Prof. Chattock and Mr. Tyndall on Pressure of the 



shape and position of each curve which is now of importance., 

 we have arbitrarily chosen a different scale of ordinates for 

 each so as to bring out these characteristics most clearly. 

 The scale of abscissae is in centimetres, and is the same 

 for all. 



For positive discharge the curve is a straight line cutting 

 the axis of z very near the origin, and very little affected a& 

 regards shape or position by the presence of the moderate- 

 quantities of oxygen we used. The distance from the origin 

 to the point of cutting is z (see Table), and its value lies 

 between 1 and 5 mm. 



For negative discharge the curve is also straight in its. 

 upper part, but in the case of low percentages of oxygen it 

 bends towards the origin, so that z w r hich is now the cutting 

 point of the straight part produced may have much higher 

 values than for positive. Those we have observed have lain 

 between 1 and 30 mm. 



In the curves the continuous lines are ruled straight, the 

 dotted portions following the experimental points where 

 necessary. 



In the earlier experiments on hydrogen by the point and 

 ring method (loc. cit. p. 87) the same sort of effects were 

 met with ; z for negative varying between 8 and 13 mm., 

 and for positive between and 2 mm. The high value for 

 negative was there attributed to a back discharge of positive 

 ions from the ring, these being supposed to meet and com- 

 bine near the ring with some of those from the point. In 

 this way the small values of dpjdz near the ring were 

 explained, and the fact that beyond a certain distance from 

 the ring the curve became straight was taken as proving 

 that the combination all occurred within that distance. 



This view is probably wrong. In the present experiments, 

 for instance, the electricity in each cub. centim. of the dis- 

 charge-tube was always considerably less than 1 E.S. unit, 

 which means that for every ion there were more than 

 10 9 molecules of hydrogen. The chances of back discharge 

 ions combining with those from the point must thus have 

 been negligibly small except quite close to the point, and 

 they must be pictured as traversing the tube practically from 

 end to end. Their effect on the wind-pressure curve will 

 thus be to reduce its slope without altering its straightness ; 

 and so, unless we arbitrarily assume the existence of two 

 kinds of positive ions, one of which combines with the 

 negatives while the other does not, we find in back discharge 

 a simple explanation of the abnormal ionic velocities, but not 

 of the large values of z Q . 



