TRANSACTIONS OF SECTION P. 363 
In importing this mechanical view of the breaking down of complex into 
simpler molecules we are not without some solid basis of facts to go upon. My 
own observations have shown that even in the solid state the crysta)line molecule 
can be broken down by purely mechanical means into the simpler units of the 
amorphous state; and, further, that the water molecules of a crystal may by the 
same agency be broken away from their combination with the salt molecules. 
Since the publication of the earlier of these observations Professor Spring has 
shown that the acid sulphates of the alkali metals may be mechanically decom- 
posed into two portions, one of which contains more acid, and the other more base 
than the original salt. It is important to recognise that in these three apparently 
short steps the transition has been made from the overcoming of the simple cohesion 
of similar molecules in contact with each other to the breaking asunder of the 
chemical union of dissimilar molecules. At each step the solid motecules appear, 
not as mere ethereal abstractions, but as substantial portions of matter which can 
be touched and handled mechanically. 
The physical properties of a gas are primarily due to its being an assemblage of 
rapidly moving molecules. These simpler and more general properties can coexist 
with, and may be modified by, the more complex relations introduced by chemical 
affinity as it occurs in compound gases and mixtures. 
It appears to me quite legitimate similarly to regard the physical properties of 
a liquid as due to its being an assemblage of rapidly moving molecules. The liquid 
system is highly condensed, and the motions of its molecules are controlled by the 
cohesive as well as by the repulsive forces. The closer approximation of the mole- 
cules may reduce their mean free path to an extremely small amount, or it may 
even cause their translatory motion to disappear, so that the whole kinetic energy 
of the liquid molecules may be in the form of rotation or vibration. 
As we can imagine a perfect gas, so also may we imagine a perfect liquid, the 
physical properties of which are as simply related to the laws of dynamics as are 
those of the gas. But the conditions of the liquid state being also those most 
favourable to the play of chemical affinity, the internal equilibrium of solutions or 
of mixed liquids must be a resultant of this affinity together with the primary 
forces of the ideal liquid state. 
An ideally perfect solution—that is, a solution the physical properties of which 
are determined solely by the number of molecules it contains in a given volume— 
must consist of a solvent and a solute which have no chemical affinity for each 
other, so that their molecules will neither associate nor dissociate in solution. Prob- 
ably only comparatively few solutions will be found which even approximate to 
this ideal perfection. But it appears to me that the study of the problems of the 
liquid and the dissolved states may be much simplified by the recognition (1) that 
the primary physical properties of liquids and solutions are due to the fact that 
they are assemblages of molecules endowed with the amount and the kind of 
kinetic energy which is proper to their temperature; and (2) that as these primary 
physical properties of the liquid and dissolved states may be masked and interfered 
with by chemical affinity, they should be studied as far as possible in examples 
where the influence of this force ig either absent or at a minimum. 
CAPE TOWN. 
WEDNESDAY, AUGUST 16. 
The following Papers were read :— 
1, Recent Developments in Agricultural Science. By A. D. Haut, M.A. 
See Reports, p. 266. 
2, Recent Researches on the Assimilatory Processes of Plants. 
By Horace T. Brown, LL.D.,. FR. 
