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(A) That two or more of the substances submitted to 
molecular influences have a more or less powerful attraction 
for one another. 
(B) That their physical condition is favourable to molecu- 
lar action. 
(C) That the molecular agent or “ catalyte” has very low 
chemical affinities for the substances acted upon. 
(D) That the molecules of the catalyte are free. 
Each of these conditions were considered separately in 
their bearing upon different molecular actions. 
(A) Of Molecular Affinity. All substances may possess 
absolutely the same amount of molecular force, but from the 
differences in the degree of approximation of their particles* 
from their varying chemical affinities, andyrom the various 
extent to which they are subject to the different affections 
of matter, they may not be able equally to exert their 
molecular power. 
The following classes of actions were noticed as instances 
of molecular affinity, and the mode in which it acts was 
pointed out : — 
1. Attraction of solids for liquids, including capillary 
attraction ; the rise of liquids in porous substances, 
and upon the surfaces of solids ; the formation of 
drops ; and some of the phenomena of cementation 
by means of colloid substances. 
2. Mixtures of fluids. — It was pointed out that in order 
that two fluids should be entirely miscible, it is 
necessary that the particles of each fluid should have 
for those of the other a greater molecular affinity 
than they have amongst themselves. 
3. The attractions of vapours and gases were illustrated 
by the distillation and diffusion of mixed vapours 
and gases. 
4. The solution of solids was supposed to be due to 
molecular affinity, causing disintegration and liqui- 
