1885. J 
Action of Chemical Forces . 
647 
to come temporarily within the sphere of power of other 
poles, and thus groups of three atoms are being constantly 
made, and broken. This residual attraction of third poles 
constitutes viscosity, and also keeps the liquid within the 
narrowest limits. The further addition of heat causes the 
residual attraction of third atoms to vanish ; the pairs of 
atoms now fly asunder under the influence of impaCt : the 
body is a gas. Each of the pairs is a molecule; and the 
attraction holding the atoms in the molecule together is not 
chemical attraction proper, as between unlike and unequal 
poles, but a concentrated remnant of the general diffusive 
attraction of cohesion which a pole in the solid exerted upon 
all others within its range of power. The manner in which 
a solid breaks up into pairs of atoms can be illustrated in a 
very simple manner. Take a strip of paper a few inches 
long and of uniform width, and cut partially across the paper 
in two places, so as to divide it into three equal segments. 
However equal in strength the connecting uncut links be- 
tween the segments may appear, it is impossible to so pull 
the segments apart that the middle one shall be left free of 
both the end ones. One of the two is sure to be a little 
stronger than the other, and the middle segment remains 
attached to one or other of the end pieces. 
The opposite nature of positive and negative poles is seen 
in the influence which the mass of the atom seems to have 
upon its affinity. The positive elements of highest atomic 
weight have the most intense affinity ; caesium is the most 
aCtive alkali metal, and lithium the least aCtive. Iernium 
(at. weight about 225) will be more aCtive than caesium. On 
the contrary, negative atoms of least weight have the most 
intense affinity ; chlorine is generally more aCtive than 
bromine, and bromine than iodine. 
The exercise of that form of attraction by poles which 
we have termed chemical attraction proper evidently does 
not prohibit the attraction of cohesion which each pole can 
exercise. The cohesive attraction survives after the affinity 
of the poles is satisfied, otherwise all compounds would be 
permanent gases. But the ratio of the two attractions is 
not constant in all poles. Hitherto our illustrations have 
principally been taken from the first two or simple cycles, 
and we have seen how closely atomicity and atomic volume 
