1. Ciliary activity is significantly influenced by chemical and physical properties of the liquid medium in which the cilia beat. 2. We studied the effect of changes in pH, ionic strength and viscosity on the ciliary beat frequency (CBF) of explants of human respiratory mucosa. 3. Optimal CBF was elicited at pH 7 . 0-9 . 0, with a marked reduction of CBF outside these limits. The CBF was well preserved at NaCl concentrations between 5 g/l (80 mmol/l) and 12 g/l (200 mmol/l), but there was rapid loss at concentrations below 0 . 5 g/l (10 mmol/l). The cilia beat best at viscosities below 1 . 0 centipoises (1 mN s m-2). Increase of the viscosity gradually decreases CBF with a significant drop at viscosities above 87 millipoises. 4. It is concluded that the above limits may fairly accurately indicate the actual physical characteristics of the periciliary environment ('sol layer') in vivo.

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The alteration of the fluorescence of the B 2Σ state of the CN radical on collisions with foreign gases has been studied by their effect on the decay. A vacuum uv nanosecond flash lamp was used to dissociate the ICN molecule with the direct formation of the CN(B 2Σ). A value for its lifetime extrapolated to zero pressure of the parent gas is 60.8±2.0 nsec. Some gases such as He, Ar, and CF4 do not give any measurable quenching cross section. Others such as Xe appear to quench by promoting the B2Σ → A2Π radiationless transition. There is a remarkable difference between the quenching effects of N2 and CO; a quasistable intermediate complex is proposed for the latter gas in analogy with the effect of these gases on Hg(3P1) atoms. Many gases appear to quench by hydrogen abstraction, e.g., H2, CH4, C2H4. In its chemical properties there is a useful analogy of B 2Σ CN with the F atom. As a consequence, in hydrogen abstraction reactions, the resultant HCN is expected to be vibrationally excited as is the corresponding HF.