December i8, 1919] 



NATURE 



401 



merits on the plates were not exactly radial, which 

 he took to mean that they were not due to gravity, 

 but to some irregular refracting medium. He further 

 said that Einstein himself regarded the shift of the 

 solar spectral lines as vital to his theory. 



Prof. Lindemann and Dr. Jeffreys agreed in thinking 

 that the experiments were by no means decisive 

 against the existence of the spectral shift. The latter 

 further stated that a medium capable of producing the 

 observed shift of the stars by refraction would reflect 

 a great deal of sunlight, whereas the plates showed 

 no trace of such matter near KTauri. 



FLOTATION PRINCIPLES OF ORE 

 EXTRACTION. 



A T the meeting of the Institution of Mining and 

 -^*- Metallurgy, held on November 20, a paper 

 entitled "A Contribution to the Study of Flotation" 

 was presented by Mr. H. Livingstone Sulman. After 

 giving a brief historical review of the development of 

 flotation as applied to ore extraction, with the problems 

 that arose in connection with successive phases of 

 the process, Mr. Sulman dealt principally with froth 

 flotation, which he ( haracteri'ed as the final link in 

 a long chain of effort. The essentials of this process 

 are that an aqueous pulp shall be agitated with certain 

 reagents which may be classified as a " froth- 

 producing " material, a "froth-stabilising" substance, 

 and a "gangue-modifying " addition. 



The explanation of flotation may be based on the 

 differences shown by various substances in the degree 

 to which they are "wetted" by water and other 

 liquids. "Wetting" is a condition of wide variability, 

 and a theorv of flotation must be based largely upon 

 the physics of wetting. The degree of wetting may 

 be influenced by the molecular porosity of the solid 

 surface, and indicated more or less quantitatively by 

 the "contact angle" made between the free surface 

 of the liquid and that of its interface with the solid. 



Reviewing the various problems encountered in 

 dealing with flotation, Mr. Sulman devoted consider- 

 able attention to the molecular constitution of liquids 

 and solids, gravitation and molecular forces, surface 

 energy and surface tension, interfacial tension v.hich 

 involves consideration of the effects of complete 

 wetting and differential wetting, hysteresis, adsorp- 

 tion, the rdle plaved by immiscible oil, and the action 

 of modifving agents such as acids. Tn this last con- 

 nection the theories of flocculation and deflocculation 

 have to be taken into account, including their elec- 

 trical relationship. Film flotation and differentia! 

 flotation receive separate attention. 



The general summary of the paper gives promin- 

 ence to the following findings : — Flotation reactions 

 result from the molecular forces acting at the sur- 

 faces of solids and liquids; these arise from un- 

 balanced molecular attractions in the surface layers, 

 which in turn are in functional relation to the 

 balanced molecular attractions constituting cohesion 

 for a solid or a liquid. Every solid or liquid, there- 

 fore, possesses excess energy at its surface, which 

 mav be exhibited in adhesion effects. Liquid-solid 

 adhesion is broadlv reciprocal to interfacial tension. 

 The degree of wetting can be relatively quantified 

 within certain limits by the contact angle made 

 between the free surface of the liquid and that of 

 the solid. Contact angles have a minimum .ind 

 a maximum value; the angular difference between 

 these values is the hysteresis of th<> contact angle, 

 which permits a wider range of equilibrium for a 

 floating particle. 



The dvnamical aspect of the subject is concerned 

 with the molecular constitution of the interfaces, with 

 NO. 2616, VOL. 104] 



the kinetic effects of molecular motion at the surfaces 

 and interfaces of solids and liquids, and with thOSe 

 in the interior of liquids. Solid surfaces are prbfaably 

 penetrable by the molecules of liquids, which enhances 

 the adhesions Ix'tvveen them ; such penetrations rniv 

 give rise to a persistent tendency for the solid to be 

 again wetted by the same liquid. Concentration- of 

 foreign molecules at the surface of a pure or homo- 

 geneous liquid (positive adsorption) reduces the 

 surface tension of the liquid and confers upon it the 

 pro[x>rty of "frothing." 



Frothing reagents useful in flotation produce ;i 

 froth with water, yet leave a partial strain (mineral- 

 adsorptive energy) at the bubble surface. The mineral 

 ad.sorption now stabilises the film, especially if the 

 mineral be minutely oil-filmed; still more so if floc- 

 culated. To be employed effectively the bubble system 

 must be disseminated throughout the mass of ore-pulp. 

 When water-strain is completely removed from the 

 surface of suspended particles, deflocculation results. 

 Flocculation is greatlv increased by mechanical agita- 

 tion, by minutely oiling the particles, and by contact 

 with air ; these are factors necessary to produce 

 standard mineralised froths. Generally, if a subst.Jilce 

 can be flocculated it can be floated. Electrical pheno- 

 mena are concomitants of minor order. Flotation 

 depends on bringing about the most advantageous 

 selective adhesions, selective adsorptions, and selective 

 flocculations between the complex of particles in an 

 ore-pulp. 



THE BRITISH ASSOCIATION AT 

 BOURNEMOUTH. 



SECTION L. 



educational science. 



Opening Address (Abridged) by Sir N/\pier Shaw, 

 LL.D., Sc.D., F.R.S., President ok the Section, 



Educational Ideals and the Ancient Universities. 



A I'KKsiDENTiAL address before the Educational Sec- 

 tion of the I^rilish Association is an undertaking that 

 might fairly daunt the bravest of those who are reall\ 

 acquainted with its difficulties. The vast range and 

 variety of the problems of education ; the enormous 

 amount of effort that is already expended upon them ; 

 the torrents of advice and criticism that are offered 

 bv those who are familiar with the details of the 

 various curricula, who know how things ought to be 

 done — if I had had time and capacity to becorne 

 acquainted with all these things, 1 suppose I must 

 have avoided the duty of making an address. It iS, 

 perhaps, the detachment of my present position from 

 anv responsibilitv for details which gives me the 

 courage to recall experiences, now twenty years old, 

 acquired during a lengthy service in various capacities 

 at Cambridge, and matured by twenty years of the 

 consciousness of the dire need of educational discipline 

 and training for those whose business it is to use 

 science in the service of the State. 



With a certain amount of assurance 1 can even be 

 glad that 1 am not in touch with the educational con- 

 troversies of the hour, and confidently trust that rtjy 

 deficiencies will be made good by the contributions 

 of those who know to the discussions which will take 

 place in the Section, but the difficulty that I cannot 

 get over just now is that, from the unavoidable circum- 

 stances of the ))resent time, a presidential address is a 

 "back number" before it is delivered, for the simple 

 reason that, according to tradition, it must be printed 

 in advance. In this particular year there is an almost 

 immeasurable gulf of experience between the time of 

 in\ appointment in 1917 and the delivery of. this 



