On the Impact of Smooth Elastic Spheres. 215 
taking these observations was that these capacities were 
unaltered during the time of any one comparison. Conse- 
quently any comparison of the readings in the different sets 
A, B, C, &e., is quite absurd, and of no value whatever. 
Mr. Walker seems to think that I tried to prove from the 
results given in my paper that @ was proportional to the 
pressure. In regard to this I might state that I made no 
such statement in that paper, nor drew any such conclusion 
from the results given there. This question has been inde- 
pendently investigated by Prof. Perrin * and by Rutherford 
and McClung +; and I would refer Mr. Walker to the papers 
where these experiments are described. I might also remind 
him that there is the question of secondary ionization from 
metals coming in here, especially at low pressures. 
I hope, therefore, that the above remarks will set aside all 
doubts in Mr. Walker’s mind as to whether the theory of 
recombination is supported by experiment. In regard to the 
question as to whether the saturation-current gives a measure 
of @, I might just remind him what is meant by saturation 
current. By saturation current we mean the current pro- 
duced by all the ions in the gas reaching the electrodes 
under the influence of the electric field before they have had 
time to recombine. Q is the total number of ions in the gas; 
therefore I cannot see how the saturation current can be 
anything else than a measure of Q. 
Cavendish Laboratory, Cambridge. 
May 20th, 1904. 
XXII. On a simple Graphical Method of treating the Impact 
of Smooth Elastic Spheres. By Cuaruus H. Lass, D.Sc.t 
S the methods of teaching mechanics are being discussed 
at the present time, it seems worth while to call the 
attention of teachers to a geometrical method of treating the 
impact of smooth elastic spheres which is so simple that it 
has probably occurred to many, although there is no 
mention of it in any of the text-books with which I am 
acquainted. 
The diagram of velocities may serve as a basis for graphical 
methods, but they are not so compact as those based on the 
diagram of momenta, since momentum is conserved on impact. 
The following method depends on the diagram of momenta. 
* Annales de Chimie et de Physique, xi. p. 496 (1897). 
| Rutherford and McClung, Phil. Trans. A, exevi. (1901). 
{ Communicated by the Author, 
