42 Proceedings of the Royal Irish Academy. 



which have now made their appearance. Fig. 5 gives two curves for negative 

 ionisation. These curves were obtained thirty-two and thirty-five days after 

 drying had begun, and they show quite clearly that no negative ion is present 

 of mobility less than 2'5. These curves run quite smoothly through the 

 region where curves obtained previously had shown discontinuities correspond- 

 ing to the four common types of ions found in moist air. On the other 

 hand, the familiar ions all show up on the positive curves. As usual, these 

 are plotted to various arbitrary zeros of current in order to exhibit them 

 apart ; but in addition it has here been found necessary to plot them to 

 different scales of current, as the negative current values are much greater 

 than the positive, and as the positive currents increase very rapidly as the 

 voltage increases. The ions of mobility roughly 2-5, 2, 1-8, 1-5, and 1'35 are 

 easily recognized. Two observations give an ion of mobility 2-3, correspond- 

 ing to an ion found twice before only faintly marked in the negative ionisa- 

 tion in moist air. 



Turning now to fig. 6 for the curves obtained at lower voltage, and con- 

 sidering the negative curves first, we have no difficulty in recognizing the ion 

 of mobility 12, which shows up as a discontinuity at about 10 volts. Instead 

 of the ion of mobility 6'6, which we had found in moist air by the air-current 

 method, we now find two ions of mobility 8 and o'6 approximately. In 

 addition, we find the ions of mobility 4:2 and 3'0 already found by the air- 

 stream method. It is readily seen also that the current begins to rise at 

 about 5 volts, corresponding to the ion of mobility 24. It is not always easy 

 to determine this voltage accurately. 



If we examine the positive curves, we find that they do not differ much 

 from the negative either as to the quantity or the character of the ionisation. 

 The ions of mobility 12, 8, 5"6, 4*2, and 3'0 are all present. The first positive 

 curves we obtained started sharply from about 10 volts, corresponding to the 

 mobility 12. After forty-two days' drying, however, we obtained a curve 

 starting quite as definitely at a voltage of 5, and thus indicating the existence 

 of an ion of mobility 24. Two curves of this character were obtained, of 

 which one is reproduced. A later curve under conditions exactly similar as 

 far as our knowledge goes failed to show this ion. The conditions for the 

 appearance of this ion are at present under investigation. 



It should be stated that in deducing mobility values from these curves, 

 we have applied certain small corrections to the critical voltages in order to 

 allow for the asymmetry of the field. 



Some hesitation might be felt in accepting as correct mobility values 

 deduced from discontinuities occurring at small values of the alternating 

 field. With this in mind, we constructed another apparatus, practically 



