The narrator speaks in the third person, but with the words, tone and way of thinking of a character, without quotation marks or "he thought": the voice of the narrator and that of the character blend.
In The Magic Mountain
a fever of matter that accompanied the process of incessant decomposition and reconstitution of albumen molecules [...]. It was neither matter nor spirit. It was something between the two, a phenomenon sustained by matter, just like the rainbow over the waterfall, and equal to the flame. [...] Illness was the licentious form of life.
The narrator sets out Hans's conclusions without quotation marks or a verb of thought, and the words "fever of matter" and "licentious form of life" sound like those of the young man reading at night. The voice of the narrator and that of the character blend.