bodies. Thus, corrosive and pungent substances may become mild
and tasteless; solids may become fluids, and solids and fluids gases.

No body will act chemically upon another body at any sensible distance;
apparent contact is necessary for chemical action. A freedom of motion
in the parts of the bodies or a want of cohesion greatly assists action,
and it was formerly believed that bodies cannot act chemically upon each
other unless one of them be fluid or gaseous.

Different bodies unite with different degrees of force, and hence one
body is capable of separating others from certain of their combinations,
and in consequence mutual decompositions of different compounds take
place. This has been called _double affinity_, or _complex chemical
affinity_.

As in all well-known compounds the proportions of the elements are in
certain definite ratios to each other, it is evident that these ratios
may be expressed by numbers; and if one number be employed to denote the
smallest quantity in which a body combines, all other quantities of the
same body will be multiples of this number, and the smallest proportions
into which the undecomposed bodies enter into union being known, the
constitution of the compounds they form may be learnt, and the element
which unites chemically in the smallest quantity being expressed by
unity, all the other elements may be represented by the relations of
their quantities to unity.

5. _Electrical Attraction._ A piece of dry silk briskly rubbed against a
warm plate of polished flint glass acquires the property of adhering to
the glass, and both the silk and the glass, if apart from each other,
attract light substances. The bodies are said to be _electrically
excited_. Probably, all bodies which differ from each other become
electrically excited when rubbed and pressed together. The electrical
excitement seems of two kinds. A pith-ball touched by glass excited by
silk repels a pith-ball touched by silk excited by metals. Electrical
excitement of the same nature as that in glass excited by silk is known
as _vitreous_ or _positive_, and electrical excitement of the opposite
nature is known as _resinous_ or _negative_.

A rod of glass touched by an electrified body is electrified only round
the point of contact. A rod of metal, on the contrary, suspended on a
rod of glass and brought into contact with an electrical surface,
instantly becomes electrical throughout. The glass is said to be a
_non-cond