charged ions are being formed. It would suggest that RuO and Ru02 are mobile surface species that can nucleate into the growing oxide. Ru03+ and Ru04+ already have substantial partial pressures and are chemically less likely to form oxide. The presence of Ru2032+ and possibly Ru203+ would imply that RuO and Ru02 are reaction partners contributing to the oxide. However, a more detailed study is needed to confirm the existence of the singly charge species. Should the high mass peak be the dimer (Ru02)2+, it would confirm the presence of diffusing and reacting Ru02 molecules analogous to the W03 species on oxidizing tungsten. The point will be scrutinized in future studies with a higher resolution instrument. From the above information, the following mechanism is suggested:

In part 1 the experimental data on the surface tensions of sodium dodecyl sulphate (S.D.S.) solutions in a number of different sodium chloride concentrations at 20", 40" and 60°C are presented. The effect of salt on the force-area curves of S.D.S. is compared with that predicted from recent theories.

SUMMARYI. The force-area characteristics of monolayers of saturated and unsaturated phospholipids have been studied.2. Interaction in phospholipid monolayers depends on the nature of the fatty acid constituents of the phospholipid, and on the temperature.3. The mean area per molecule in mixed films of cholesterol with (I,2-distearoyl)-3-lecithin and (I,2-didecanoyl)-3-1ecithin at 22 ° practically followed the simple additivity rule.4-A condensing effect of cholesterol was evident with films of (I-stearoyl-2-lauroyl)-3-1ecithin; (I,2-ditetradecanoyl)-3-1ecithin, (I-stearoyl-2-oleoyl)-3-1ecithin and the corresponding ethanolamine analogue, as well as with (I,2-dioleoyl)-3-1ecithin at 22 °. At 5 ° the condensation effect with (I,2-ditetradecanoyl)-3-1ecithin was much reduced.5. The very expanded films of synthetic lecithins and phosphatidyl ethanolamines containing linoleic and linolenic acid showed no appreciable condensation effects with cholesterol.6. The behaviour of the mixed cholesterol-phospholipid films is governed by a number of factors, including Van der Waals' interactions, configurational entropy effects and alterations in the structure of water adjacent to the monolayers. These factors depend on chain length and degree of unsaturation.