  A report by Brillouin (from Perrin's laboratory) on the rate of adsorption of
`granules' to a glass plate [\textit{Ann. Chim. Phys.} 27 (1912) 412--23]
prompted Marian von Smoluchowski (MvS) to interpret the data in terms of his
newly developed theory of restricted Brownian motion. Placing an adsorbing wall
at $x=0$, he modelled the particle concentration $n(x,t)$ as that solution of
the diffusion equation which vanished at the wall, a boundary condition (BC)
hereafter called SBC. A gaping discrepancy between his theory and Brillouin's
data elicited a suggestion from MvS (that a particle might not adhere to the
wall on every impact), but no further action -- other than that of applying his
theory to spherically symmtric systems. In a paper written before, but
published shortly after MvS's untimely death [\textit{Proc. Roy. Acad. Amst.}
20 (1918) 642--58], H. C. Burger erected a new and sturdier framework, which
led him to an alternative BC, $D(\partial n/\partial x)_{x=0}=\varkappa
n(0,t)$, applicable to a surface with an arbitrary absorption probability
($1\leq\varepsilon\leq 0$); a fallacy (that subsequently claimed more victims,
including the present author) prevented him from deducing the correct
expression for $\varkappa$. Burger's approach became ``The Road Not Taken'',
while the SBC became the cornerstone of colloidal coagulation and bimoleculer
reaction kinetics. Burger's approach (but not the ABC) was partly rediscovered
by Kolmogorov, and used by Sveshnikov and Fuchs.The emended version of Burger's
BC is shown here to coincide with that deduced from the Klein-Kramers equation
[\textit{Phys. Rev. Lett.} \textbf{49} (1982) 304--07; \textit{J. Chem. Phys.}
\textbf{78} (1983) 2710--12] and the Lorentz model of random flights
[\textit{J. Phys. Chem.} \textbf{86} (1982) 4750--56].
