I read this article with utter awe, because we now know what a momentous event this was. But it was even more incredible than what I had known, because it appears that Onnes' lab had discovered more than just superconductivity, but without realizing it.
The experiment continued into the late afternoon. At the end of the day, Kamerlingh Onnes finished with an intriguing notebook entry: "Dorsman [who had controlled and measured the temperatures] really had to hurry to make the observations." the temperature had been surprisingly hard to control. "Just before the lowest temperature [about 1.8 K] was reached, the boiling suddenly stopped and was replaced by evaporation in which the liquid visibly shrank. So, a remarkable strong evaporation at the surface." Without realizing it, the Leiden team had also observed the superfluid transition of liquid helium at 2.2 K. Two different quantum transition had been seen for the first time, in one lab on one and the same day!
That just plan incredible!
Of course, while this was during the throes of the early formulation of quantum mechanics, no one knew at that time that superconductivity (and superfluidity) was the clearest manifestation of quantum effects.
An amazing article. Highly recommended.