It was the more necessary for him to postpone any open breach with Athens, because the Olynthians had conceived serious alarm from his conquest of Amphipolis, and had sent to negotiate a treaty of amity and alliance with the Athenians. Such an alliance, had it been concluded, would have impeded the farther schemes of Philip. But his partisans at Athens procured the dismissal of the Olynthian envoys, by renewed assurances that the Macedonian prince was still the friend of Athens, and still disposed to cede Amphipolis as her legitimate possession. They represented, however, that he had good ground for complaining that Athens continued to retain Pydna, an ancient Macedonian seaport.[490] Accordingly they proposed to open negotiations with him for the exchange of Pydna against Amphipolis. But as the Pydnæans were known to be adverse to the transfer, secrecy was indispensable in the preliminary proceedings, so that Antiphon and Charidemus, the two envoys named, took their instructions from the Senate and made their reports only to the Senate. The public assembly being informed that negotiations, unavoidably secret, were proceeding, to ensure the acquisition of Amphipolis—was persuaded to repel the advances of Olynthus, as well as to look upon Philip still as a friend.[491] [490] Thucyd. i. 61, 137; Diodor. xiii. 49. Pydna had been acquired to Athens by Timotheus. [491] This secret negotiation, about the exchange of Pydna for Amphipolis, is alluded to briefly by Demosthenes, and appears to have been fully noticed by Theopompus (Demosthenes, Olynth. ii. p. 19. s. 6. with the comments of Ulpian; Theopompus, Fr. 189, ed. Didot).