S' (Continued from page 54) 1696: J. F. H. Custers and J. C. Riemersma, The texture of straightrolled and of cross-rolled molybdenum, Physica 's-Grav.12, 195-208, 1946. . With the aid of pole figures the textures of straight-rolled and of crossrolled molybdenum are determined. The pole figures thus obtained show that earlier authors described these textures in a too simple way; they are at least twofold. For example, after cross-rolling, there is found besides the so-called (100) [HO] tecture ((~OO)parallel to the rolling plane, and [110] parallel to the rolling direction) a second texture, which is rotary symmetrical around the normal to the rolling plane, and which has a (111) plane parallel to this plane. The texture of straight-rolled molybdenum turns out to be in good agreement with the texture of straight-rolled iron, as determined by K urdjumow and Sachs; the texture of cross-rolled iron is not known. 1697: N. G. de Bruijn, A combinatorial problem, Proc. Kon. Ned. Akad. Wetenschappen Amsterdam 49, 758-764, 1946. A Pn-cycle is defined as an ordered cycle of 2n digits 0 or 1 (i.e. a series of such digits placed on the circumference of a circle), such that the 2n possible sets of n consecutiv~ digits of that cycle are all different '(as a consequence, any ordered set of n digits 0 or 1occurs exactly once in that cycle). Posthumus studied these cycles in connection with a practical problem of telecommunication, and was led to the conjecture, that the number of Pn-cycles be equal to 2N where N = 2n-I_n. This conjecture is proved to be correct, as follows from a theorem concerning a special type of networks. Another application of this theorem is mentioned too. (Continued on page 79)

AbstractAccording to a method developed by de Boer powder samples baked out in vacuo are exposed to iodine vapour which is adsorbed at the powder surface. The amount adsorbed is determined by a simple chemical titration. The precautions necessary and the reproducibility of the observations are discussed in detail. Surface reactions and the effect of milling in a ball mill can clearly be traced by corresponding changes in the adsorption.

Es wird eine einfache Methode beschrieben, um dünne gleichmässige Schichten grosser Dichte und Haftfähigkeit einer gepulverten Substanz auf metallische, oder elektrisch leitende Unterlagen aufzubringen. Die Substanz wird elektrophoretisch aufgebracht, und zwar beruht die Methode auf (a) dem Gebrauch von Suspensionen der mechanisch zerkleinerten Substanz antstatt Kolloid‐Lösungen, und (b) der Verwendung geeigneter polarer organischer Flüssigkeiten als Dispersionsmittel.

AbstractIn a previous paper the London‐ v. d. Waals forces between spherical particles have been calculated as a function of the diameters and the distance separating the particles. On the basis of these data the probable magnitude and range of these forces in colloidal systems is discussed in this paper.The “range”, defined as that distance at which the energy of interaction equals the kinetic energy of Brownian motion, is found commonly to be of the order 0.06 to 0.2 times the diameter of the smallest particle: the size of the second particle is relatively of little importance.Comparing with experiment it is demonstrated that the adhesive forces observed under various circumstances are of the order of magnitude theoretically expected.Finally it is maintained that our concept of the range of a force will depend on the size of the particles considered. Different definitions of the range are compared with each other. The discussion illustrates that a correct comparison of observations of adhesive forces made by different methods or on different scales cannot be made without theoretical knowledge of the nature of the forces acting.

Frequently we experience the existance of adhesive forces between small particles. It seems natural to ascribe this adhesion for a large part to London-v.d. Waals forces. To obtain general information concerning their order of magnitude the London-v. d. Waals interaction between two spherical particles is computed as a function of the diameters and the distance separating them. A table is calculated which enables numerical application of the formulae derived. Besides approximations are added, which may be used when the distance between the particles is small. In a separate section it is investigated how the results must be modified, when both particles are immersed in a liquid. Here we are led to the important conclusion that even in that case London-v. d. Waals forces generally cause an attraction.

AbstractColloid experiments are frequently interpreted on the assumption that attractive and repulsive forces are acting between the particles. In the paper below the consequences of this assumption are analysed from a theoretical point of view. It is found that the phenomena which may arise must logically be divided into two classes. These two classes show a very striking resemblance with the phenomena observed in lyophobic and lyophylic colloids respectively. It is also easily seen that the two classes of phenomena are not definitely separated, but that intermediate cases may exist. This is again in close agreement with the result of observation.Strictly, however, the therms “lyophobic” and “lyophylic” do not apply to our classes of phenomena; instead of this a distinction between “reversible” and “irreversible” phenomena is proposed and it is maintained that, by combining this classification with the common distinction between lyophobic and lyophylic colloids, a more satisfactory system of colloids and colloid phenomena is obtained.

AbstractCorresponding to views developed in the first paper a formula is given, which roughly estimates the total energy of interaction between two colloidal particles as a function of their distance apart: this formula contains the particle charge E and the electrolyte concentration c as two independent parameters.The properties of a colloidal system are discussed for arbitrary values of E and c. The E‐c plane obtained by plotting E and c as abscis and ordinate in a rectangular system of axis is found to be split up in different portions; in one region a sol is stable, in another it is flocculated, while in intermediate regions more complicated conditions prevail.A physical change in the sol is represented by a simultaneous variation of E and c, that is by a curve in the E‐c plane. With the aid of such curves a variety of phenomena viz. peptisation, reversible and irreversible flocculation, and thixotropy are discussed in a clear and simple way.