This seems due to a vaporisation of the spirit within the tube.
"Himalayan Journals V2."
J. D. Hooker
When a little of the vapour is condensed, this portion parts with latent heat of vaporisation, increasing the tension of the remainder, or raising its point of saturation, so that before the weight descends any further, this heat has to escape from the cylinder.
"The Birth-Time of the World and Other Scientific Essays"
J. (John) Joly