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With a book of the weight and calibre of Adhesion and Adhesives it is of more than passing interest to know its intended purpose. The editors, N. A. De Bruyne of Aero Research Ltd., England, and R. Houwink of Rubber‐Stichting, Holland, in their preface describe the work as ‘… a symposium by specialists’ which ‘collects in one volume information about the scientific and technological aspects of adhesion’.

CELLULOSE is Nature's strong material. It is the chief constituent of cotton flax and wood. Wood can be turned into sugars by treatment with hydrochloric acid Bergius process) and by certain termites; horses and cows break down the cellulose in grass into sugar before digesting it. So it is not surprising that the cellulose polymer is built up of what are practically molecules of a glucose (“Barley Sugar”). Each β glucose unit is twisted about its axis through 180 deg.; the combination of two such units makes up that is called a cellulose unit which has the structure shown in Fig. 2. The cellulose polymer is a long, straight chain made up from these cellobiose units, and each chain probably contains about 70 such units.

WOOD though in many ways an attractive structural material has the disadvantage of being water absorbent. In itself this characteristic would be of minor significance were it not for the fact that it is accompanied by considerable swelling at right angles to the axes of the wood fibres. Great interest is being shown at the present time in the possibility of reducing this swelling by the use of synthetic resins. In this article the possibility of preventing swelling by such means is discussed and it is concluded that complete immunity from swelling could only be attained at the expense of the strength of the wood. The article gives an original analysis which enables the magnitude of swelling to be predicted and the expression derived is shown to be in agreement with experiment.

THE marked dependence of the strength of metals on temperature, and on rate of loading at high temperatures, can be explained by assuming that above the equi‐cohesive temperature rupture takes place by plastic flow in a non‐homogeneous medium, consisting of rigid crystallites weakly cemented together. Although the stress concentrations required by the Griffith theory must still be operative above the equi‐cohesive temperature, it is suggested that they produce intergranular flow, rather than the elastic separation that occurs at temperatures below the equi‐cohesive temperature. A theory is developed based on the assumption that the strain energy at rupture reduces the energy of activation of the flow process, and the theory is shown to be in numerical agreement with the experimental results, if the energy of activation of the flow process is about one seventh of the latent heat of evaporation per gram atom. Values of the cohesive strengths and of the stress concentration factors are also derived.

THE life of an engineer is largely a life spent in solving problems of a practical kind; and so his attitude to nature differs from that of the scientist. Confronted with a particular material an engineer will first ask in what way he can use its particular and special properties; whereas the scientist will want to know how those particular and special properties arise. But a good engineer must have knowledge of the scientist's conclusions, so I hope engineers will not think I am wasting time by trying to describe in these articles why wood behaves like wood and why rubber is “rubbery.” I have tried to keep the engineer's point of view in mind, and I have not attempted to give anything more than an outline of the reasons why a given material has its special and peculiar properties.

In this article the author discusses the suitability of existing materials for aircraft structures and shows that a hypothetical material derived by expanding an aluminium alloy would greatly simplify the structure of fuselages

SYNTHETIC resins reinforced with fahric have proved in actual service to be not only resistant to disintegration from shocks and vibration but also to have remarkable freedom from “notch sensitivity.” In order to investigate this property measurements were made of the energy absorbed by such materials under torsional oscillation; it was found that the energy absorbed was greater than that of any other comparable materials (such as wood or metal).

WITHIN the last year or so we have learned to glue metals together with a strength which brings this method of joining materials into competition with riveting, at least in the thin gauges used in the aircraft and motor industries. Apart from this new extension of gluing to the metal working trades synthetic adhesives have already revolutionized the woodworking industries. This revolution is due to the superior quality of the resulting products and the increased rate of output made possible by the intrinsic high speed of setting of synthetic adhesives aided by such novel methods as high frequency heating, infra‐red heating and the like. In aircraft in particular the “weather‐resistance” of synthetic adhesives has largely removed the disadvantages of wood construction, due to the use of casein glues, so much in evidence in the first winter of this war. It may be said therefore that gluing has been raised from the status of a useful but humble convenience of daily life to a process of engineering significance. But before engineers can use gluing in the fabrication of structures they must be provided with data sufficient to enable thorn to compute the strength of the joints, and we at Aero Research Ltd. have therefore endeavoured to find a simple relation between the strength of a lap joint and its geometry. Such a simple relation is found in “the joint factor” which is defined3 as the square root of the thickness of the sheet divided by the length of the overlap.

SYNTHETIC AND OTHER PLASTICS CELLULOSE and rubber are made of long chain molecules already built up for us by nature, but in recent years we have learned to make synthetic polymers with physical characteristics not unlike those of natural polymers; the variety of such synthetic polymers is already somewhat bewildering, and it is quite certain that chemists have only made a beginning in an exploration that promises to be as successful as anything achieved by the classical organic chemists of the nine‐teenth century. Like many other branches of knowledge the subject is easier to understand if not treated in the chronological order in which the discoveries have been made. We will therefore begin with a description of the vinyl polymers rather than the older and better‐known condensation resins.

CELLULOSE is a polar material held together by the strong secondary forces emanating from its OH groups. It can be swollen, dissolved or cemented together (according to circumstances) by a polar liquid like water or alcohol, while it is unaffected by non‐polar liquids such as benzene or toluene. It is rigid and relatively inextensible and strong.