The structure of thin films. Part VII.-Critical evaporation phenomena at low compressions

The measurements of compression and area on monomolecular films of fatty substances on water surfaces, described previously (1), were limited in accuracy by the disturbing effect of the jets of air necessary in the apparatus employed for measuring compression, for the purpose of preventing escape of the films past the barrier confining one end of the films. We think the error in measurements of compression with this apparatus may have been as large as half a dyne per centimetre in extreme cases, and as the apparatus was used under fairly accurately reproducible conditions, it was of the nature of a constant error. The reality of the lowest portion of the compression-area curves was therefore open to doubt. We have now designed an apparatus in which the gap at the ends of the floating barrier is completely closed by very thin strips of gold ribbon, placed vertically in the water, and soldered to the ends of the float and to brass pieces pressed tightly against the sides of the trough. This arrangement can be made leak-proof and very sensitive. In order to measure the force on this float, at a given position, fine silk fibres connect the float to (a ) an optical lever system consisting of a mirror suitably damped with an oil bath, mounted on a light torsion wire, which is in effect a practically frictionless bearing, and (b ) a fine torsion wire provided with an accurately graduated divided head, whose divisions can be calibrated in absolute measure by hanging weights on a small balance mounted on the wire. Full details of the apparatus are given at the end of the paper.