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LETTER XXXV.

The experimental method of philosophising unknown

to the ancients---its superiority to the ancient method
of speculation-physical principles not yet ascertained.

LETTER XXXVI.

Of the Loadstone---how far its properties were known

to the ancients---Modern discoveries concerning it---
Theories to explain its phænomena---wherein it re-
sembles, and wherein it differs from electricity.
LETTER XXXVII.

0
No bodies in nature perfectly solid-the impenetrability

of matter denied by some philosophers—all the parts of
nature connected---The integral parts, and primary
agents, imperfectly known.

LETTER XXXVIII.

Of combustible substances---opinions concerning their

formation ---characters of them---their powerful ac-
tion within the earth---origin of coal.

LETTER XXXIX.

Of phænomena caused by the action of fire and water

within the earth ---causes of volcanic eruptions---
cause of the explosion of gunpowder, and of aurum
fulminans---Earthquakes from internal fires---from
electricity

LETTER

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