ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 471 



such as copper. The powder is mixed with soap into a paste, and can 

 now be bought in tubes such as are used for oil colours. The powders 

 obtained by calcination of the above materials are treated with nitric 

 acid, washed thoroughly, and allowed to settle in distilled water con- 

 taining 0*2 p.c. of ammonia. When treating 10 grm. of powder in a 

 litre flask, yoths of the liquid are siphoned off at the following intervals 

 of time : £-hour, 1 hour, 4 hours, 24 hours, and 8 days. The third de- 

 posit is useful in p. dishing hard metals, such as iron, but the fifth and 

 last deposit affords the best polishing powder. The soap preparations 

 are applied in the ordinary way to discs of wood or metal covered with 

 skin or cloth and capable of being revolved at high velocity, the whole 

 operation of polishing proper being carried through by their aid in 

 5 minutes. 



For examining and photographing the polished and etched specimens 

 under the Microscope, M. le Chatelier proposes the use of monochromatic 

 light, such as that derived from an electric arc in mercury vapour, with 

 suitable screens between the source of light and the object to be illu- 

 minated ; but it appears doubtful whether enough light can be obtained 

 in this way lor high magnification. 



M. le Chatelier proposes to shorten the search for typical alloys 

 by melting together two superposed layers, each consisting of a pure 

 metal, the lighter one being on the top. If no alloys are formed of 

 greater density than the heavier metal, and the crucible is allowed to 

 cool undisturbed, a culot can be obtained which, on being sawn through 

 vertically, shows a complete gradation from one pure metal to the other, 

 passing through the whole series of alloys, which can then be studied 

 in one specimen. 



New Reagents for the Micrographic Study of Carburised Iron.* 

 — MM. Osmond and Cartaud have, after numerous trials, discovered a 

 reagent of more constant strength than liquorice juice for etching iron. 

 They recommend a dilute solution of nitrate of ammonium (2 parts in 

 weight of the crystallised salt to 100 parts of water). A piece of 

 parchment spread tightly over a smooth board is soaked with this 

 solution, and the polished surface of the specimen is rubbed upon it 

 until sufficiently etched. It is not necessary to add any sulphate of 

 calcium. When the parchment becomes dry a little water is poured 

 over it. The results are exactly those produced by the extract of 

 liquorice, and the various constituents are identified as follows: — 



(1) Pearlite by the unequal depth of etching of its two components, 

 and sorbite by its coloration, varying from light yellow to dark brown. 



(2) Troostite by its yellow, brown, or blue bands merging into one 

 another. 



(3) Martensite by its characteristic needles, which are the better 

 defined the less the carbon in the metal. 



(4) Ferritebyits division into grains, and the heterogeneous appear- 

 ance of these grains. 



(5) Austenite, and especially cementite, remain unacted upon. 

 There are difficulties in the application which can be overcome by 



perseverance and experience. Concentration of the solution, pressure, 

 speed, and quality of the parchment, all affect the result. 



* Metallograpliiat, 1900, pp. 1-3. 



