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two for instance are (1) the molecular theory, (2) the dynamical 
theory of heat. 
1. On the molecular constitution of matter, and its pro- 
perties. The chief properties, as distinct from the allied forces 
of matter, are those with which we are now concerned, namely, 
cohesion and elasticity; for convenience sake often treated 
separately. Cohesion is the force that holds together the 
particles of a bit of granite, and ranged in the same class is 
another property, that of elasticity, e.g., the elasticity of granite. 
With both of these, the forms of energy in nature have to do. 
Such forms are heat, light, and electricity or electrical energy. 
These forms of force can all in great measure be studied without 
express reference to any one special kind of matter. In the 
ultimate structure of matter the unit is the molecule, and though 
the chemist inclines to the use of the word atom, other scientists 
regard the atoms as constituents of the molecule. Dr Daubeny, 
a name grateful to our Cotteswold Naturalists’ Society, enlarges 
in his excellent work on the Atomic Theory (1850, ed. ii.), upon 
the atom and its chemical affinities, and adopts the word ‘ mole- 
cule’ with a limited sense; but this was published nearly 40 
years back. Now, the molecule as a constituent of matter, 
although its existence cannot be detected by the most delicate 
instruments, is held to be something of a wide sphere of 
action, one molecule exerts upon another a force which is 
mutual, and to which is due the cohesiveness of matter. The 
forces are named intermolecular, and those forces applied to 
the molecules from without are known as impressed forces. 
The whole subject is replete with difficulties that stand in the 
way of developing the theory analytically. Take for instance, 
the absolute bulk and dimensions of molecules; the law of 
distribution of their mean positions in their natural state; the 
law of intermolecular force; the manner in which it depends 
upon and varies with both the configuration and the temper- 
ature; the limits of its sphere of action; and lastly, the 
connection between mean configuration, period, and amplitude 
of vibration, and the temperature. On almost all these points, 
there is at present absolute ignorance. For instance, as to the 
