M: p. 166); or even to the
rotation of the earth (M: p. 332). However (M: p. 129), he
also mentioned that a terrella would revolve by itself!

But before reaching this point in his argument, Gilbert digressed to
classify the different kinds of attractions and motions which the
elements produce. In particular, he distinguished electric attraction
from magnetic coition, and pointed out the main features of electrical
attraction. Since the resultant motions were different, the essential
natures of electric and magnetic substances had to differ.

Gilbert introduced his treatment of motion by discussing the
attraction of amber. All sufficiently light solids[85] and even
liquids,[86] but not flame or air[87] are attracted by rubbed amber.
Heat from friction,[88] but not from alien sources like the sun[89] or
the flame,[90] produce this "affection." By the use of a detector
modeled after the magnetic needle, which we would call an electroscope
but which he called a "versorium,"[91] Gilbert was able to extend the
list of substances that attract like amber.[92] These Gilbert called
"electricae."[93]

[85] M: pp. 78, 82, 84, 86.

[86] M: pp. 78, 89, 91.

[87] M: pp. 89, 95.

[88] M: pp. 83, 86.

[89] M: pp. 81, 86, 87.

[90] M: pp. 80, 81, 86, 87.

[91] M: p. 79.

[92] M: pp. 77-78, 79.

[93] M: p. 78. The definition Gilbert gave of an electric
in the glossary at the beginning of his treatise was not an
experimental one: "Electricae, quae attrahunt eadem ratione
ut electrum."

Possibly as a result of testing experimentally statements like that of
St. Thomas, on the effect of garlic on a loadstone, Gilbert discovered
that the interposition of even the slightest material (except a fluid
like olive oil) would screen the attraction of electrics.[94] Hence
the attraction is due to a material cause, and, since it is invisible,
it is due to an effluvium.[95] It must be much rarer than air,[96] for
if its density were that of air or greater, it would repel rather than
attract.[97]

[94] M: pp. 86, 91, 135.

[95] M: pp. 96, 135.

[96] M: p. 89.

[97] M: pp. 90, 92, 95.

The source of the effluvia could be inferred from the properties of
the electrics. Many but not all of the electrics are transparent, but
all are firm and can be polished.[98] Since they retain the appearance
and properties of a fluid in a firm solid mass,[99] Gilbert concluded