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The purpose of the present investigation was to establish the value and suitability of the scanning electron microscope for research on the mechanism of fracture and cohesive failure of the structure of wood. Wood samples, mainly consisting of various pines, were stressed to the point of failure in tension and shear parallel or perpendicular to the main fibre direction. The rate of loading and the temperature were varied between wide limits. The fractured surfaces were studied and gave the following results :(1) In tension parallel to the grain, fissures developed between the microfibrils in the cell walls causing the wood to rupture perpendicular to the longitudinal axis of the fibres. At both high and low temperatures the matrix, in which the microfibrils are embedded, and to a much lesser extent the middle lamella between the fibres failed by plastic flow when the external load was applied very slowly. Under such conditions strands or bundles of microfibrils were pulled out and fractured in an irregular pattern. Cohesive failure in tension caused by sudden impact was characterized by limited slippage of microfibrils and a near brittle failure of the embedding matrix. A remarkable clean cleavage approaching a true brittle failure was observed when the wood was stressed rapidly perpendicular to the grain.(2) Cohesive failure when the wood was exposed to shear along the grain and tension across the grain, showed clearly that the weakest part of the ultrastructure of wood under such conditions was the bonds between parallel strands of microfibrils which were pulled out with little resistance from the matrix. However, the adhesion between the middle lamella and the cell wall or the middle lamella itself failed when the wood was exposed to cleavage parallel to the grain. If the process of cleaving was carried out extremely fast, as in the machining of wood, the rheological properties of the middle lamella did not allow for plastic deformation with subsequent failure, and the bulk structure of the wood tissue behaved almost as a homogeneous material.The use of the scanning electron microscope for the study of the structure of wood under stress and the mechanism of its cohesive failure can supply information which seems to be unobtainable by any other method.

SUMMARY The use of the scanning electron microscope for the study of the changes in the structure and ultrastructure of weathered wood was evaluated in a study of wood exposed to natural weathering for several hundred years. Stereoscan photographs taken of the surface structure of wood revealed in detail the mechanism of the structural breakdown, which could not be found by using the light microscope or the conventional transmission electron microscope. The gradual and very slow deterioration and ultimate destruction of the middle lamella, the various layers of the cell wall and the cohesive strength of wood tissue as well as the wood fibres are illustrated by a serious of photographs. Since no preparative technique can be used which would interfere with the surface of the weathered wood and due to the deep three‐dimensional change in the structure, the scanning electron microscope was found to be the ideal instrument for the study of the structure of weathered wood at both high and low magnification with excellent resolution of detail.

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The spreading of cellulose derivatives at the air-water interface has been examined and it has been confirmed that, whereas ethyl cellulose spreads readily and almost completely on water, cellulose acetate and nitrate spread very imperfectly. Limited increases in area per glucose unit for the acetate were obtained by removal of incompletely dispersed aggregates, by spreading from dilute solution, by varying the solvent and by use of NaOH, HCl, or salts in the substrate. On solutions of urea and guanidine hydrochloride (G. HCI), ethyl cellulose gives a slightly increased area per glucose unit, but cellulose acetate and nitrate give areas up to 80 % and 190 % greater than on water. With increasing acetyl

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