The decay rate of the ice-forming properties of silver iodide smoke has been studied under the condition of ultraviolet light irradiation. The smoke from its generator was, at room temperature, introduced into a smoke-chamber (52×30×28cm
3) and exposed, through a sheet of polyvinyl chloride making one of the walls of the chamber, to an ultraviolet light source (mercury lamp); measured, small amounts of the smoke were withdrawn, at frequent intervals, from the chamber by means of a syringe, and discharged into supercooled cloud in a ice-box (-12°C); the number of ice-crystals thereby produced and immediately, falling was counted by the naked eye; the same procedure was taken as well with the smoke previously mixed with water vapor, ammonia or hydrogen sulfide and subsequently subjected to the irradiation of ultraviolet light. The particle-size distribution of the smoke was estimated from the electron microscope photograph taken with shadow-casted sample to be ranging from 0.01μto 0.4μin diameter, the maximum frequency of the distribution in size appearing at about 0.07μ; the initial concentration of the smoke in the chamber was controlled to yield about 103 ice-particles per cm
3 through all the runs of experiment.
The results obtained are shown in condensed form as follows: Water vapor, ammonia and hydrogen sulfide are all effective in keeping the nucleation agency of silver iodide smoke from the deactivating action of ultraviolet light. The effectiveness of water vapor as protective agent increases rapidly with its concentration and lasts almost the highest value at relative humidities of about 60% or more, and this tendency is found to be more remarkable with ammonia; the ability of hydrogen sulfide falls between them.
The above results were analyzed by making use of a decay-rate equation derived by taking into consideration of (1) the reduction in nucleating activity of smoke due to ultraviolet light irradiation, (2) the sedimentation as well as adhesion of particles to stirrer and walls of smoke chamber and (3) the coagulation of particles owing to collision between them, and the specific rate of photolytic deactivation (kp) was estimated by applyingg the equation to experimental data.
With the intention of inquiring into the mechanism of the protective action of water vapor, the studies were carried out, next, on the adsorption of water vapor on finely divided silver iodide powder at room temperature by gravimetric method. It was found through the runs that the adsorption of water vapor is negligibly small in the range of relative humidities less than 60% or thereabouts, and increases sharply as the relative humidity goes over 70%, this circumstance being in striking contrast to the interrelation between the decrease of kp and the water vapor content of smoke. It appears, moreover, from the results of the adsorption experiment that the particles of silver iodide smoke would be covered, in an atmosphere of relative humidities of 70% and upwards with an adsorption layer of water of several decades of molecules thick.
The results of the present experiments on adsorption of water as well as photolysis lead to the conclusion that the adsorption layer might turn down the transmission of ultraviolet light and prevent iodine, produced by photolysis, from being scattered and lost into surroundings, and these processes might stand for the protective action of water vapor.
The effect of ammonia is supposed to be due to the chemisorption on silver iodide or, more probably, to the formation of complex-compounds of the type AgI•nNH3 (n=1/2, 1, 3/2, 2, •••); the same would happen to hydrogen sulfide.
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