Inspissate

To thicken, especially by boiling, evaporation, or condensation; condense; To become viscous

This is from The Curse of Doone by Sidney Horler (1928). I am reading it on the bus and enjoying it immensely.

From the book:

Here was the imposing balustrade gallery, which, running around the top of the large hall, connected with the main staircase.

Even in daytime, so dark was the panelling and so ill-lit this particular part of the house that Cicely always hastened past it on her way downstairs. It had an atmosphere of inspissated gloom.

Every nerve strained, she listened. She had not been mistaken – it was a noise – a stealthy sound like a person walking cautiously.

The fear which had been dormant in her mind now assumed an actual shape; someone was either ascending or descending the broad staircase.

That someone must be after the secret!