In this extract, DeForest while noting that the crystal detector marked an important advance in simplifying radio reception, dismissed the popular idea that portable personal telephones might soon be available, although he was optimistic that some sort of radio-telephone service would eventually be developed. Decades later, the silicon crystal detector would be refined into the transistor, which would finally make lightweight portable telephones practical.
 
Journal of the Franklin Institute, June, 1907:

ELECTRICAL  SECTION.
(Stated  Meeting  held  Thursday,  February  21,  1907.)
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Recent  Developments  in  Wireless  Telegraphy
DR.  LEE  DE  FOREST.
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page 464:

    Another class of oscillation detectors utilizes the peculiar electrothermic qualities of crystalline or pseudo-crystalline substances, notably silicon and psilomelane. Silicon may be used without a local battery. The unequal heating effect produced by the oscillation at the junction between silicon and a copper electrode gives rise to feeble local currents which can operate the telephone receiver, to produce an audible signal.
    This class of receivers is certainly the simplest and least costly imaginable, and marks an approach towards that Utopian state of affairs so much heralded by the popular press when each of us will carry a responder in his vest pocket, a telephone on his head and with steel-rod umbrella in his hand, and lead soles upon his shoes, shall be within telephone reach of every other unfortunate similarly equipped!