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136 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

separately ; very few of them would bear a careful aualysis, not
being based upon direct proof but upon certain principles or
beliefs which, in themselves doubtful enough, become still more
so when applied to the teeth. For example, he says, " The
condition and arrangement of the several layers (composing the
skin) are incontestably those of an electro-voltaic series," conse-
quently the pulp-vessels, together with the dentine and enamel,
also form an electro-voltaic series.
At present the electric theory is to be regarded rather as a
curiosity than as a theory of dental decay. Its promulgation had,
liowever, a secondary action which for a time was ver}' strong,
and which is still felt to some extent at the present day ; it may
consequently not be out of place to consider it at some length.
Prompted by Bridgman's exposition, experiments were under-
taken to determine the position of dentine in the electro-chemical
series. These experiments resulted in the establishment of the
following scale: Electro-negative — gold, amalgam, tin, gutta-
percha, dentine, oxychloride of zinc -[-electro-positive. From this
it follows that the highest electro-motive force, or the strongest
electrical action, is generated by a combination of gold and den-
tine; combination of amalgam and dentine yields a weaker
still weaker is that produced by a combination of gutta-
force ;
percha and dentine. On the strength of these and similar ex-
periments Chase ^"^ maintained that a tooth filled with gold would
necessarily soon become carious again on the margins of the cavity,
wherever the acid secretions of the mouth constantly bathe the
fillino- and the borderino; tooth-substance. A tooth filled with
amalgam succumbs to this electro-chemical process less rapidly,
while one filled with tin still longer escapes destruction. The
comparative rapidity with which teeth filled with gold, amalgam,
and tin are destroyed is expressed by the numliers 100, 67, and
50. On the other hand, a tooth filled with oxychloride of zinc
becomes electro-negative by contact with this material, and is
tlierefore protected from the efl:ects of acids (which are also
electro-negative).
Chase also succeeded in obtaining experimental evidence in
support of these calculations. He prepared })ieces of "ivory of
equal shape and size, bored a hole in each, and filled the holes