I happened to visit Zürich a couple of weeks ago, and it was tempting to see places, where Albert Einstein lived. Einstein lived in Zürich three times. First, when he was a Eidgenössische Technische Hochschule (ETH) student in 1896-1900, second, between 1909 and 1911, when he served as an adjunct professor, and third in 1912-1914, when he became a full professor at the ETH, after returning from Prague. There are several addresses related to Einstein in Zürich. One of the most interesting is the house at Moussonstrasse 10 (formerly 12), where Einstein lived since October 1909 until March 1911.

The Moussonstrasse 10 house (right), where Einstein lived in 1909-1911 (my photo)

Einstein’s papers of this period, in English translation, can be seen here. In my opinion, an interesting paper is A. Einstein. Bemerkung zu dem Gesetz von Eötvös [Comment on Eötvös’s Law] *Annalen der Physik* (ser. 4), 34, 165–169.

This paper deals with the surface tension (surface energy) of liquids. Einstein was trying to relate the surface energy to the enthalpy of boiling and then, using the so-called Trouton’s rule, to the entropy of vaporization. According to the old-fashioned Trouton’s rule, the entropy of vaporization of any liquid is about 85–88 J K^{−1} mol^{−1}. How can this value be the same for any substance? Well, according to Boltzmann’s understanding of entropy, the entropy of vaporization has to do more with geometry (with the change of the phase space volume after vaporization, to be more precise) than with the type of molecules or their interactions.

Can we conclude anything meaningful from all this? Keep in mind that this is just a zeroth approximation. Today we know much more about the nature of surface tension.

Together with my student Rahul Ramachandran, we discussed modern concepts of the surface tension in our paper M. Nosonovsky and R. Ramachandran “Geometric Interpretation of Surface Tension Equilibrium in Superhydrophobic Systems” *Entropy* 2015, 17(7), 4684-4700; https://doi.org/10.3390/e17074684. We criticize over-simplified views on surface energy. In particular, please see the discussion of Eq. 11 (which follows the same line of reasoning relating the surface energy to the enthalpy of vaporization, as Einstein did in 1910) and the following paragraphs.