6 Transactions of the Society. 



Having briefly considered the laws controlling - the structures of 

 slowly cooled iron-carbon alloys, it will perhaps be well now to con- 

 sider how these structures may be made evident under the Microscope. 



The specimens for examination are generally prepared by remov- 

 ing sections from the original sample about three-quarters of an inch 

 square and a quarter of an inch thick. The surface is carefully ground 

 on a series of emery papers, mounted on carefully " trued " wooden 

 discs attached to a lathe head, using ultimately the finest grades which 

 can be produced. As the finest commercial papers are much too 

 coarse, it is necessary to prepare the final papers oneself. This is 

 done by washing the very finest slime from the best flour emery, 

 mixing it with a solution of egg-albumen in water, and brushing it 

 on paper specially free from grit. The paper is then allowed to dry 

 in a cupboard, great care being taken to exclude all dust. 



Prof. H. le Chatelier has recently succeeded in shortening the 

 polishing operation by means of a new method of preparing the 

 powders. The operation which has been until now considered the 

 most tedious has become the quickest. The most important point in 

 the preparation of polishing powders, and also of emery powders, is to 

 obtain an absolutely accurate classification with regard to the size of 

 the particles in each case. The levigation method previously men- 

 tioned is altogether defective, even when carried out in the laboratory 

 with all the precautions indicated by M. Osmond. Prof, le Chatelier 

 claims, however, that perfect classification is obtained by the washing 

 method used by M. Schloesing for the analysis of kaolins.* 



Briefly, his method is to obtain some suitable polishing powder, 

 such as the alumina obtained by calcining ammonia alum or even flour 

 emery, and treat it with water containing ■ 1 per cent, of nitric acid, 

 in order to dissolve the carbonate and sulphate of lime and other salts 

 that might be present. The mixture is stirred occasionally for several 

 hours, and is allowed to settle, which it will then do rapidly. The 

 powder is then washed several times by decantation with distilled 

 water until it will no longer settle rapidiy. The conditions are now 

 favourable for levigation, which operation is facilitated by adding two 

 cubic centimetres of ammonia to each litre of water, which helps the 

 finest particles to remain in suspension. Decantation is now resorted 

 to at intervals of fifteen minutes, one hour, four hours, twenty-four 

 hours, and eight days. The first deposit contains all the grains un- 

 suitable for polishing ; the second is not very homogeneous, but may 

 be used to start the polishing : the third constitutes a good polishing 

 powder for hard metals such as iron and steel. It is, however, the 

 fourth deposit that constitutes true polishing powder. Instead of 

 waiting eight days for this last deposit, it may be thrown down im- 

 mediately after the removal of the twenty-four hours one by neutralis- 

 ing the ammonia with acetic acid, when the whole of the particles will 



* See Prof. H. le Chatelier's p iper in the Bulletin de la Socictc a" Encouragement 

 pour V Industrie Nationale, Sept. 1 1900. 



