488 APPENDICEStheory' [E20]. In 1927 they wrote a joint paper on the problem of motion in gen-
eral relativity (see Chapter 15) [E21]. Another acknowledgment [E22] indicates
that Grommer was in Berlin at least as late as 1928. Funds for his support came
in part from the Kaiser Wilhelm Institut [K2]. While in Germany, Grommer
worked on the preparation of a mathematics and physics textbook in Hebrew for
use in high schools in Palestine. Weizmann had authorized financial support for
this project [R3]. In the late 1920s, Grommer accepted a university position in
Minsk. In 1929 he wrote to Einstein from Minsk that some of his lectures were
given in Russian, others in Yiddish [Gl]. He was later elected to the Bielorussian
Academy of Sciences in Minsk, d. 1933, Minsk.- Paul Ehrenfest. b. 1880, Vienna. PhD in Vienna in 1904 with Boltz-
mann. Ehrenfest first met Einstein in Prague in 1912. Their deep friendship
lasted until Ehrenfest's death. In 1922 they wrote a joint paper on the Stern -
Gerlach effect, in which the pre-quantum mechanical difficulties of understanding
this phenomenon are clearly demonstrated [E23]. Another joint paper, written the
next year, deals with an extension of Einstein's earlier work on spontaneous and
induced emission and absorption of radiation to two-photon states, with an appli-
cation to the Compton effect [E24]. From 1913 to 1933 Ehrenfest was professor
of theoretical physics in Leiden, d. 1933, Amsterdam, by his own hand. 'Paul
Ehrenfest, in memoriam,' one of Einstein's finest and most moving pieces of prose,
was written to honor a friend and 'the best teacher in our profession I have ever
known.'* In this note Einstein shows his great sensitivity to the fate of physics and
physicists. - Hans Muhsam ** b. 1876, Berlin. In 1900 Muhsam passed his final med-
ical examinations, started a private practice, and became a staff member at the
Jewish Hospital in Berlin. He first met Einstein in 1915. 'At that time, his name
was little known in lay circles' [Ml]. The meeting came about because of a chance
encounter between the Miihsams and Elsa Einstein. On that occasion Muhsam
told Elsa that he had heard of Albert Einstein and that she, Elsa, had a famous
name. When Einstein heard of this, he became curious and got in touch with the
Miihsams. A friendship developed. The men would go on Sunday hikes during
which they discussed physics and also medical and biological problems [S4].
In 1923 Einstein and Muhsam wrote a joint paper on the experimental deter-
mination of the permeability of filters [ E26]. The purpose of the experiment was
to find the maximum diameter of colloidal particles capable of permeating a given
rigid membrane. The membrane consists of the walls of a tube that is open at one
'Einstein's article on Ehrenfest was originally published in the Almanak van het Leidsche Studen-
tencorps in 1934. Its English version is found in one of his collections of essays [E25].
**I obtained most of the biographical information about Muhsam from letters by him and his wife,
Minna, to Carl Seelig. These letters are now in the Historisch-Wissenschaftliche Sammlung in the
main library of the ETH in Zurich.