"Silverly" is a word. Its examples in the OED go back to 1595 and span the whole period since.
The "which" refers to the time defined by the adverbial clause "when he talks about something that excites him". Pronouns, including the relative ones, have antecedents -- they refer to something already mentioned.
[ As a general hint, try to keep that in mind as you study English further. In fact, please allow me to point out that this was probably the source of your difficulties with the passage about scientific changes a few days ago -- you were not correlating the "it" in the clause you sought help with with the "embarrassment" that stood just before it and was its antecedent. ]
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