It is nearly a year now since we commenced the systematic investigation of our local soil clays in the North-East of Scotland by the powder method of X-ray analysis. In Aberdeenshire, one of the most wide-spread Soil Associations is the Insch Association, and this is the one I want particularly to talk about to-day. An association consists of soils derived from similar parent material, and the parent material of the Insch Association is till derived from basic igneous rock—mainly hypersthene gabbro, but sometimes with olivine also present.

The investigations on clays progressing at the Macaulay Institute are fundamentally connected with pedological studies. The general aim of these investigations is to provide evidence which we believe will give us a better insight into the fundamental differences between the soil types as mapped in the field and provide us with an explanation of certain of their characteristics and properties. Within the last year a systematic investigation has been commenced into the clay minerals associated with each different soil type in Aberdeenshire. The methods being used are X-ray examination and differential thermal analysis. A brief account of the X-ray work has been given by Mr G. F. Walker.

The function of the bond in moulding sand is that it should make the sand sufficiently plastic for moulding and give it sufficient strength in the green (moist) and dry states after compaction to retain its shape and later to support the weight of the metal. It must do this while still leaving the sand permeable to gases, and it must further retain its strength in contact with the hot metal for a sufficient time for it to solidify, after which it should be sufficiently weak to be readily removable from the finished casting.

Clay mineral research was started at Wagingen in 1935 by Dr F. A. van Baren. Later Dr J. Ch. L. Favejee continued his work. Some papers were published on X-ray work, based on Dutch clays. The clay fractions of Dutch clays are mixtures of quartz, illite, kaolinite and montmorillonite in varying proportions. Favejee and Hardon analysed also a number of characteristic soil colloids from Java. The lateritic soils contain kaolinite; the very heavy black clays are montmorillonite, the terra-rossa soils on limestones contain (meta) halloysite; the same mineral was found in the (podzolic) mountain soils. In the true residual soils no mixtures of clay minerals were observed.

Montmorillonites from various parts of the world have been tested. The main interest was focussed on the colloidal sections of the clay from which the heavy impurities were separated by dispersion and sedimentation. Use was made of the gold-shadow technique first described by Wyckoff to estimate the thickness of plates from the angle of shadowing.

Investigations of clay minerals are being actively pursued in the Fuel and Physics Departments of the University; while the investigations are being carried on independently of each other, a close liaison is maintained between the two laboratories and the special techniques developed in each are at the service of the other.

Clay minerals possess the peculiar characteristic that they normally occur as very small crystals. We cannot, therefore, afford to ignore the contribution that colloid science can make to the study of clay minerals. Many colloidal systems are stable because the particles are electrically charged. There is, of course, electrical neutrality of the system as a whole, but there is an electric separation within the system whereby electricity of one kind (it may be a positive or negative, depending on the system) is anchored for the time being to the particles while the complementary charge exists as ions (counter ions they are often called). Some of the counter ions must be regarded as in solution in the suspending medium (normally water) for it is their mobility relative to the particles which gives rise to electrical conductivity and to electrophoresis, electro-endosmosis and streaming potential.

I am going to speak about the results which we have obtained on nickel hydroxide gels. These experiments have been undertaken as a preliminary to the synthesis of certain clays; the intention is to study systematically the conditions of synthesis starting from hydroxides, while varying certain physico-chemical factors; especially the age of the gels.

This work has so far been concerned mainly with montmonllonite. We have been attempting to study the behaviour of this mineral in contact with different reagents. It was with this general idea that Miss R. Glaeser took up the study of equilibria in the vapour phase with a series of organic substances. The first results are expected soon. The principal difficulty arises from the fact that it is necessary to operate in an anhydrous medium: as is well known it is difficult to eliminate all the water fixed by montmorillonite without partially altering the mineral.

At the present time two hypotheses have been proposed to explain the formation of clays. One of them supposes that these minerals arise from the crystallisation of a gel due to the breaking down of the original mineral; one would thus have schematically:

Original mineral → Gel. → Clay.

The application of the de Saladin-Le Chatelier differential thermal analysis method to the identification of soil clays has given rise to a certain number of difficulties. In fact, it is possible for these clays to be altered either in situ by the action of atmospheric and biochemical agents or in the laboratory during their extraction. Mr Henin and I have been investigating what influence chemical agents can exert on the thermal diagrams of different types of clays. The treatments have been effected by normal solutions of cations in the form of chlorides, carbonates and nitrates, or by N/10 acid or basic solutions.

These authors are studying the influence of pH and particle size on sedimentation volume, and on the velocity with which it is reached. They are using fractions from > 1.2 μ to < 0.3 μ, obtained by sedimentation, and buffer solutions of pH 1, 3, 5, 7, 8, 9, 10, as well as distilled water. For all pH's the volume of sedimentation and the time taken to attain it rise with decreasing particle size. For a given fraction the volume of sedimentation rises in the pH's near neutrality (7 and 8) and in distilled water, depending partly on the ions present, their nature and concentration, in the buffering solutions employed. The thixotropy and the base exchange capacity rise parallel with the sedimentation volume.

This is normally measured in terms of resistance to shear, which is a function of the internal friction and cohesion of the soil. The internal friction is primarily a characteristic of the coarser soil fractions and results from the interlocking of the particles; its value is a function of the normal stress on the soil. The cohesion of a soil is produced by forces binding the particles together, and is independent of the normal stress. These binding forces are accounted for partly by the, surface tension at the air-water interface of the water films surrounding the particles and partly by the intermolecular forces acting between films of water adsorbed on the surfaces of neighbouring particles.

One of the most interesting and practically most important questions regarding the behaviour of clays, and particularly montmorillonites, is concerned with the water binding mechanism in relation to the mechanical properties, elasticity and plasticity.