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The profound biological significance of the clotting of blood needs no emphasis here; the vast number of studies of the process which have been carried out testify sufficiently to its importance, and also show clearly the incompleteness of our knowledge of the underlying mechanisms.4The properties of fibrinogen solutions and of fibrin clots may be profoundly modified in many ways by suitable variations in the conditions of preparation. Thus a wide variety of products is obtainable. In the subsequent papers of this series, the properties and uses of several of these, namely, fibrin clots, fibrinogen plastics, fibrin films, and foams made of fibrinogen and thrombin, will be considered. Here, we shall discuss the properties of certain preparations of protein fractions from human plasma, which may be obtained in active and stable form and which possess specific action on the various aspects of the blood clotting mechanism.The fundamental feature of this mechanism is the transformation of a solution of the protein fibrinogen into the rigid insoluble fibrin clot. This transformation does not occur spontaneously in solutions of suffi8ient purity, but is nor-

For several decades bacteriologists and epidemiologists have been interested in the survival of micro-organisms in the airborne state. Studies by many investigators, recently summarized by Wells (1955), have produced data concerning certain aspects of bacterial survival or death, but the information has not generally sufficed to permit quantitative analysis when a fluid environment was changed in the airborne state.

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