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London Archaeologist, 16 (6), 142-153

Light scattered by natural crystalline quartz is studied as a function of temperature and wavelength, and of time after change of temperature. Anomalous scattered flux is found to be divisible into two distinct fractions. The first fraction is highly anisotropic and orientation-sensitive and is consistent with the existence of `needles' lying parallel to the c axis. This fraction reacts at a different rate to change of temperature, depending on whether the temperature is raised or lowered. A semi-quantitative interpretation is given in terms of substituted impurity atoms which form atmospheres near edge dislocation lines lying parallel to the axis. On lowering the temperature there is a drift to the atmospheres, and on raising the temperature there is a (faster) diffusion away from them. A single diffusion coefficient of the order 10-20 cm2sec-1 is derived for both processes. The equilibrium temperature dependence allows an estimate of the mean binding energy of an impurity atom to the dislocation line to be made which agrees with prediction from the theory of Cottrell and Bilby. The second fraction is not readily explained in terms, directly, of defects, and it is tentatively suggested that defects are indirectly responsible through perturbation of the spectrum of lattice vibrations, leading to a modification of thermal scattering.

The pairs of observations at non-normal incidence which specify the various reflection methods of determining the optical constants n and k are compared as to their arithmetical sensitivity to changes in n and k and hence as to their theoretical suitability. Only the simple methods which do not require polarimetric analysis are considered. The comparison of sensitivity is done by inspection of charts prepared from computed tables and which cover a wide range of n and k. A previously unnoticed property of the Fresnel equations at 45° incidence allows any method, not just one in which a ratio of reflectivities is measured, to be employed without a standard reflector. Experimental factors as well as theoretical sensitivity are considered, and the use of the Brewster angle as one observation is proposed. An exact analysis of the Brewster condition has been made, which leads to explicit equations applicable to two proposed methods. The distinction between the Brewster angle and the principal angle of incidence, which are often confused, is emphasized.

Single crystals of the cubic form of sodium chlorate are studied in growth from aqueous solution. An interferometric technique measures, via refractive index, concentrations and gradients of concentration contiguous to the crystal. The rates of advance of individual faces are measured and compared with predictions derived from the Nernst supposition, namely, that the concentration is saturation at a crystal face and that the facial rate of advance depends only on diffusion. It is found that faces sometimes grow at rates consistent with this supposition, but that on other occasions smaller rates of growth are observed, including instances of complete stoppage of growth. It is also found that individual faces can change discontinuously from one rate of growth to another. The observation of Berg, that concentration and normal concentration gradient are functions of position at the face, is developed, and empirical equations are obtained for the functions which connect the rate of growth with the concentration and gradient distribution along the face. To account for the advancing surface remaining plane, notwithstanding the non-uniformity of normal concentration gradient along it, it is necessary to suppose a transport of material along the face. Some observations on dissolution have been made, and a non-uniformity of concentration along a dissolving face is found and discussed.

The intensity of light scattered at 90° to the incident beam has been investigated as a function of wavelength for a specimen of colourless, apparently perfect, quartz. The wavelength function indicates that, in addition to the light scattered by thermal fluctuations of density, to be expected in any crystal, however perfect, there is a contribution from some real-structure possessing a linear dimension of about 3000 A. Previous work by the present author pointed a similar effect in NaCl crystals, in which a dimension of about 1500 A was derived and a preliminary interpretation of the effect was offered in terms of a theoretic real-structure first suggested by Born and later discussed by Furth. It is now pointed out that the formula given by Born does lead to a dimension for quartz about twice that for NaCl. But since more than one kind of imperfection could produce this `size effect' a discussion is given of imperfections from this point of view. Two kinds come into consideration: (i) clusters of the correct size randomly embedded in the lattice, (ii) a three-dimensional net throughout the lattice, made up of surfaces of disorder enclosing regions, of the correct size, perfection. It is argued that the former will produce discrete appearances the beam of light, detectable by ultra-microscope, and that an entirely diffuse beam of light indicates their absence. This is the case with several crystals which nevertheless exhibit the size effect, and it seems therefore that the latter alternative, the net, has to be accepted. This conclusion is discussed with reference to present knowledge of subgrain structure as well as to the theory of Born and Furth.

The total intensity of light scattered at 90° by specimens of natural and synthetic NaCl single crystals has been investigated as a function of wavelength from 6600A to 2500 A. The scattered intensity is found to be of an order 40 times greater than that expected from thermal theory, and the wavelength function is not inverse fourth power throughout the spectral region studied. Therefore only a small part of the scattering can be attributed to thermal fluctuations and the bulk is attributed to a submicroscopic block structure of the crystals. From the wavelength function an estimate has been made of the linear dimension of the blocks, and this is found to differ little from specimen to specimen and from place to place in one specimen; it is of the order 1500 A. Thus the above-mentioned structure seems to be an intrinsic feature of the real structure of single crystals.

Crystals of Tipula iridescent virus (TIV)3 diffract light just as ordinary crystals diffract X-rays, so that a crystallographic study is possible using visible light. The conditions of growth of TIV crystals have been investigated and a method developed for preparing large crystals from the microcrystalline centrifuged virus pellets. Optical observations using monochromatic light have been made on crystals oriented parallel to the face of a perspex cell. The virus concentration in the normal wet crystal in calculated from the measured refractive index to be 17 ± 4% w/w. The virus particles are packed in a face-centered cubic array, with an interparticle spacing of 2500 ± 30 A , a distance nearly twice the diameter of the frozen-dried particle. The nature of the crystals is discussed and it is concluded that the (hydrated) virus particles in the crystal are not in contact but are separated by large distances of water (∼ 500 A). The crystals are thus probably held together by long-range forces operating at a distance comparable with the size of the particles themselves. The TIV crystals seem to represent the first clearly established instance of an ordering of iso-dimensional colloidal particles in three dimensions.

London was the administrative centre for and largest city in Roman Britain. After centuries of\ud excavation, Londinium is one of the best understood cities in the Empire. London is also home\ud to one of the most exceptional collections of craft and agricultural tools in the Roman world.\ud These objects represent a wide range of practices, including woodwork, metalwork,\ud leatherwork, masonry, agriculture, and animal husbandry. Due to excellent preservation in\ud waterlogged contexts, many are in exceptional condition.\ud This thesis brings together c.837 metal (mostly iron) tools from multiple collections, many of\ud which have not been published before. Using a combination of detailed typological study and\ud theoretical perspectives on technology and practice, this thesis provides an innovative insight\ud into society and economy amongst the working people of a Roman city; a diverse population of\ud locals, immigrants, specialists and amateurs.\ud A typological discussion identifies these usually neglected objects with reference to French and\ud German literature, highlighting new types for the first time in Britain, and demonstrating a close\ud connection to Continental working practices. These artefacts are then used as the basis for a\ud discussion of craft and agricultural practice in London, focussing on how tools were made, used\ud and discarded. Tools are synthesised with evidence from finished objects, waste, tool marks,\ud structures, epigraphy, iconography and classical sources. This discussion reveals that craft\ud practices were highly specialised, with numerous distinct professions which cannot be\ud accurately condensed to ‘woodworking’ or ‘leatherworking’. Tools were used in working\ud practices which shaped peoples’ lives; either limiting their opportunities of social mobility or\ud providing avenues to express pride in their work. Several industries were controlled in part by\ud the state, or by Roman citizens. Finally, a detailed contextual analysis reveals high levels of\ud metalwork consumption, with deposition in the Walbrook valley largely reflecting rubbish\ud disposal, and not ritual activity.