Chapter of Accidents
(A ). Unforeseen events. To trust to the chapter of accidents is to trust that something unforeseen may turn up in your favour. The Roman laws were divided into books, and each book into chapters. The chapter of accidents is that under the head of accidents, and metaphorically, the sequence of unforeseen events.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894