JM: and I wonder why he said "chemical reagent," since a fall would only entail mechanical, not chemical, consequences. Unless...?
MR: Nice catch, Jansy. My thought is that Nabokov is being precise here in making Kinbote imprecise. According to Webster's 2nd, a chemical reagent is "any substance which, because it takes part in certain reactions, is used in detecting, examining, or measuring other substances, in preparing material, in developing photographs, etc." Kinbote seems to be describing a catalyst, as someone pointed out. But if a reagent is used to detect a new or otherwise hidden substance, we might reverse Kinbote's notion and say that Kinbote himself is the substance and the Dante-esque incident of the Shades stuck in the ice is the reagent--that which first brings Kinbote into view of the Shades (though they do not seem to detect him at that moment). The description of Shade strenuously grimacing and Sybil talking fiercely to him may imply that there is more wrong at that moment than a stuck car. In the audio file of VN reading this passage at the 92nd Street Y, VN says that Sybil was "fiercely talking into him," a description which brings Shade's interior into the equation more forcefully than the description as it appear in PF.