As regards pressure and temperature, I have not yet 

 found any effect produced by variation of these conditions. 

 The quantity RP/T appear to be a constant, P being the 

 pressure and T the absolute temperature. This implies that 

 the stopping power of an atom or molecule is independent of 

 P and T. Examples of the fact that RP is constant while 

 T is constant are given in the paper "On the Recombination 

 of Ions in Air and other Gases." The following experimen- 

 tal result will serve as an illustration of the fact that R is 

 proportional to T when P is constant. The ionisation vessel 

 filled with air was raised to a temperature of 90 "" C, the 

 pressure being 763 mm. R was then found to be 5'98. Now, 

 when P = 760 mm., and T-20° C, R = 4-83. 



. 1 4-83 X 363 X 760 -, r.r.x 

 And = l-OOo. 



5-98 X 293 X 763 



It has, of course, been pointed out by several observers that 

 the ionisation effects of radium are largely independent of 

 pressure and temperature and of physical and chemical condi- 

 tions generally. This, however, does not cover the present 

 statement, which refers to the stopping power of the atom, 

 a quantity which has not previously been the subject of 

 measurement, so far as I am aware. 



To sum up, the range of the a particle in a given gas is 

 in the first place easily measured, and in the second place 

 simply related to the constitution of the gas and independent 

 of its state. It is a delightful contrast to some other radio- 

 active quantities, and often gives a welcome foothold in diffi- 

 cult places. 



The quantity I is in quite a different class. It is much 

 more difficult to measure accurately, as I have already des- 

 cribed. But there appears to be a more important difference 

 in that the total ionisation of a gas is not simply dependent 

 on the weights of the atoms of which it is composed. Mole- 

 cular structure counts for something. Perhaps also the vari- 

 ous atoms do not yield ions in simple proportion to the energy 

 spent on them, but this point is not yet sufficiently clear. 



An example of this want of uniformity has already been 

 given in the paper to which reference has been made. It was 

 shown that RI in ethyl chloride is much greater than RI 

 in air. The difference must be yet a little greater than that 

 shown, as no allowance was made for the small 

 quantity of air mixed with the heavy gas. Again, RI m 

 standard pentane (mostly C.^Hia) is nearly half as much again as 

 in air, and the same is almost certainly true of benzene 

 (Cells) ; but this vapour is harder to treat than pentane, since a 

 high temperature is necessary. Generally speaking, the more 



