The aim of this study is the investigation of the instability of the two-di
mensional pow in an open, side-heated container of rectangular cross-sectio
n. For long containers the pow is quasi-two-dimensional if the thermocapill
ary Reynolds number and the Rayleigh number are small. On an increase of th
e temperature difference the pow becomes three-dimensional. We measure the
onset of the three-dimensional pour, its structure, and periodicity.