496 



NA TURE 



[April 15, 1922 



The Properties of Powders 



CONSIDERABLE interest attaches, both on the 

 scientific and on the technical side, to the 

 study of powders which are sufficiently fine to differ 

 markedly in their properties from massive crystals, 

 while they are sufificiently coarse to differ equally 

 widely from colloidal suspensions. One important 

 property of powders, namely the caking of salts (a 

 phenomenon which was responsible for the disastrous 

 explosion at Oppau), was discussed at a joint meeting 

 of the London Section of the Society of Chemical 

 Industry and of the Faraday Society on March i, 

 1920. This has now been followed up by a joint 

 meeting of the Faraday Society and the Oil and 

 Colour Chemists' Association, held at Burlington 

 House on March 9 last, when the material presented 

 was sufficiently abundant to call for an adjourned 

 discussion on March 23. 



The principal subject discussed was the grading of 

 powders by elutriation, a process which has proved 

 of great value to the geologist and to the agriculturist, 



of the Finer Constituents of Sedimentary Rocks," in 

 which the geological applications of elutriation are 

 described. One of the principal problems here is to 

 devise a method of summarising the mechanical 

 analysis of a sediment, containing particles of many 

 different sizes, in such a way that the results can be 

 expressed by means of ope or two numbers. The 

 use of a single number is impracticable, since in addi- 

 tion to the fineness of the material, its uniformity 

 must be represented by a separate coefficient. A 

 satisfactory solution appears to have been provided 

 by Dr. H. A. Baker, who makes use of the term 

 "equivalent grade" to express the average of the 

 diameters of the particles, whilst a " grading factor " 

 serves to express the deviation of the particles from 

 the average. Prof. Boswell has had much experience 

 in the practical application of elutriation, particularly 

 in the mechanical analysis of the sands and rocks used 

 in glass-making, and his notes on the difficulties and 

 errors encountered in the process are of considerable 



-Elutriation of Barytes. (x 50.) 



as well as to the manufacturer of pigments and of 

 food products where artificial grinding is required in 

 order to produce minute subdivision. 



The formal papers presented to the meeting were 

 four in number. Prof. Lowry and Mr. L. P. MacHat- 

 ton, in a paper on " The Grading of Powders by 

 Elutriation," submitted a new series of experimental 

 data as to the diameter of the particles of barytes 

 and of quartz which are just lifted by a vertical 

 current of water at velocities ranging from 4 to 8 mm. 

 per second (Fig. i). The data for barytes are more 

 concordant than those for quartz, probably because the 

 particles of barytes are, in the main, cleavage-frag- 

 ments of fairly uniform shape, whilst in the case of 

 quartz the natural conchoidal fracture produces much 

 more irregular particles. In the case of barytes it 

 was possible to show that the use of a vertical tube 

 one inch in diameter lifts particles which are 5 per 

 cent, smaller than when a half -inch tube is used ; but 

 the grading is also much more uniform as a result cff 

 the more uniform velocity of the water in the tube. 

 The temperature-coefficient was also measured and 

 shown to correspond with a decrease of 0-4 per cent, 

 only in the diameter of the particles for each degree 

 of rise of temperature ; and an empirical relationship 

 was deduced between the velocity of the water and 

 the size of the grain of barytes lifted by it. 



Prof. Boswell presented a paper on " The Separation 



NO, 2737, VOL. 109] 



value. Dr. J. W. French, who has made use of 

 water-separation for the grading of emery and car- 

 borundum for use as polishing powders in grinding 

 lenses, contributed to the same meeting a paper on 

 " Abrasives and Polishing Powders for Glass." 



Dr. R. S. Morrell, the president of the Oil and 

 Colour Chemists' Association, opened the discus- 

 sion, by emphasising the value of elutriation to colour- 

 users, as a method of controlling the materials which 

 they purchased from the pigment-makers and grinders. 



The adjourned discussion was opened by Dr. C. A. 

 Klein of the Brimsdown White Lead Company. 

 While Prof. Lowry had been working with a range 

 of sizes down to about 0-07 mm.. Dr. Klein's 

 work had dealt with grades in which this was more 

 nearly the maximum than the minimum size of the 

 particles ; they therefore presented greater experi- 

 mental difiiculties, more particularly as a result of 

 flocculation. In addition to describing a number of 

 points in connection with the practical use of the 

 elutriator. Dr. Klein stated that a specification had 

 actually been put forward by a user of pigments who 

 was calling for the supply of some hundreds of tons 

 of a product in which the largest particles would have 

 a diameter not exceeding o-i mm., whilst the average 

 size of the particles was not to exceed 0-027 mm. 

 This specification was being worked to and material 

 conforming to it could be supplied. 



