34 PRESIDENT’S ADDRESS—SECTION A, 
closer. But we must cease to multiply illustrations. In a 
thousand different ways the atomic conception seems to be re- 
inforcing itself as necessary and valid for the practical work of 
physics and of chemistry. Hach discovered relation between 
chemical constitution and physical property becomes an instru- 
ment facilitating further investigation, and so rapid is the march 
of progress that it is difficult to conjecture what will be the limit 
of our control over matter through the application of atomic and 
molecular theory. 
31. No Finality in Atomie Exrplanations—When one con- 
templates the great advances of physical knowledge, it might 
appear at first as if the mystery of matter and of its properties 
was in a fair way to be cleared up. NRelatively simple concep- 
ticns apparently exhaust their significance. Still closer observa- 
tion, however, in general introduces new difficulties or reintro- 
duces the old ones. The kinetic theory of gases as it left the 
hands of Bernouilli was equal to explaining the phenomena 
observed by Boyle. But there is no rigorously-deduced kinetic 
theory to-day equal to the interpretation of all the known facts 
connecting temperature, pressure, and volume. The Newtonian 
law of gravitation answered well until the precision of observa- 
tion showed the impossibility of retaining it as an exact expres- 
sion (a) of the observed facts. The application of the idea of 
“ close-packing” seems necessary to explain many of the phee- 
nomena of crystallisation, but how is the diffusion of solids into 
solids to be understood under that conception? We cannot play 
fast and loose with such doctrines as the conservation of mass 
and conservation of energy; to raise any question as to their 
validity is apparently to abandon everything. Yet how are 
Landolt’s experiments showing loss of weight in reacting sub- 
stances to be understood (5)? The difficulties yet to be explained 
are not trivial ones. For example, we have supposed that at 
the absolute zero manifestations of energy would disappear. The 
indications of the kinetic theory of gases seemed to be supported 
by other facts of an equally sienificant character, but that sup- 
port has now failed. As we approach the sup posed lower limit 
of temperature, phenomena are presenting themselves which may 
give us pause. The general inactivity of chemical substances at 
low temperatures, of course, is in agreement with our mechanical 
view. What, however, is the interpretation of the conspicuous 
brightening of the sulphide and iodide of mercury observed by 
Kreuz at — 181.4 C. to be? 
The test of the truth of any theory is its applicability to the 
whole range of observed facts. It is not sufficient if it is incon- 
sistent with some; it must be consistent with all. Is it to be 
(2) See sec. 40 hereinafter. 
(b) E. g. .08 to .025 mgm. in 1€0 gm. of silver sulphate and ferric sulphate reacting in 
sealed U-shaped tubes. Zeits. fiir Phys. Chem. 12. 1. (1893.) 
