286 Transactions of the Society. 



Method 2. — Instead of regulating the temperature of the bath 

 it may be heated until the surface of the solder begins to form 

 yellow films. Each specimen, preferably of the dimensions 20 mm., 

 by 10 mm. by 5 mm., is, after warming and rubbing with a cloth, 

 held at one end with a pair of tongs, and the under surface of the 

 other end is immersed in the highly heated metal. In one minute 

 or less the tinting will be complete, but it will be graduated in 

 colour between grey at one end and pale yellow at the other ; the 

 intermediate part passing through the whole gamut of colouring. 

 The specimens are removed when the central parts have assumed 

 a brown colour. 



Treated in this way the phosphorised portions will be dark 

 brown on a yellow ground, or blue on a brown ground. 



Method 3. — The specimen is heated rapidly until uniformly blue, 

 and when cold is immersed in water containing a one-thousandth 

 part of nitric acid. The films covering the phosphorised parts will 

 be dissolved in advance, and if the acid treatment is stopped at the 

 right moment it is possible to have white phosphorised areas on a 

 brown or blue matrix. This method gives very satisfactory results, 

 but many failures to obtain the exact development may follow the 

 first attempts. It is sometimes advisable to rub the developed 

 specimen with moistened chamois leather before drying with a hot 

 blast of air (pi. V. fig. 5). 



Method 4. — Instead of floating the specimens on the surface of 

 liquid metal, they are placed into a jacketed copper chamber 4 in. 

 in length and 1 in. square, which is surrounded, excepting at one 

 end, with heavy mineral oil, maintained at a temperature of 245° C. 

 A drawer is fitted into this, and into it the metal sections are 

 placed. The tinting by this method of heating is more under 

 control than by the first described, and it is easy to locate the parts 

 highest in phosphorus even in steel castings containing under 

 O'Oo p.c. of that element. 



Iodine Etching. 



This method is based on the fact that a very dilute tincture 

 of iodine in potassium iodide corrodes the portions lower in phos- 

 phorus relatively more rapidly than those containing more of that 

 element. 



The necessary reagent contains 1 gramme of iodide and 0*1 

 iodine per 500 c.cm. alcohol and 50 c.cm. water. 



The polished specimens are immersed in this and are examined 

 from time to time. When it is seen that some portions remain 

 brilliantly white on a dull ground, they are removed, washed 

 with water and alcohol, and dried in a current of hot air. 



In longitudinal sections of rolled steel after this treatment 



