276 Transactions of the Society. 



Special care is necessary in the preparation of this block, for upon 

 it the polishing is finished. On the top of the block is placed a 

 small disc of cloth of exactly the same diameter as the upper area 

 of the block. Upon this is sprinkled pretty thickly a covering of 

 about one gramme of diamantine powder, a preparation of calcined 

 alumina manufactured by A. Guyot Dupold, Tocle, Switzerland, 

 which is quite as satisfactory as the calcined alumina prepared as 

 directed by Professor H. Le Chatelier, and it has the advantage that 

 it can be readily obtained from any jeweller at a small price. This 

 diamantine powder, having been placed upon the cloth disc, a larger 

 piece of cloth is placed over it and the ring pressed home over 

 the cone. Arranged in this way, any of the larger particles of 

 powder are prevented from passing upwards through the pores of 

 the cloth, and only the finest portions reach the upper surface 

 and are utilised in the polishing. 



Professor Arnold has independently found this method of 

 procedure to be very useful. 



Many metallographers prefer to use large blocks or wheels for 

 polishing, but in my experience it has been found that with 

 smaller ones there is less danger of dust getting on the cloth, and 

 the apparatus is more convenient and less cumbersome. 



In the many designs for polishing apparatus, shown at this meet- 

 ing, it will be observed that some are fitted with a series of blocks 

 in which all the necessary grinding and polishing surfaces are close 

 together, and there is no necessity of changing the blocks. On the 

 other hand there is a machine w r ith larger blocks to suit those who 

 prefer them, and there is also a single table with interchangeable 

 discs. All these machines can be fitted with the traversing 

 specimen holders, so that polishing becomes practically automatic. 

 I am under great obligation to the makers for allowing these to be 

 exhibited this evening. 



Selection of Specimens. 



When a metallographer is called upon to make an examination 

 of a metal structure, which has broken or failed when in use, it is 

 most important that all particulars should be provided him, with 

 exact details as to the nature of the strains and stresses applied, 

 where they were applied, and whether, or not, any local distortion 

 of the metal substance has been produced in the machine shop, or 

 when in practical use. 



In selecting the position from which specimens shall be taken, 

 the metallographer must be largely guided, by the information he 

 receives. All fractures or failures in metals have initial starting 

 points, and it not infrequently happens that the seat of weakness 

 is located exactly at this point, and possibly nowhere else. A 



