Phosphorised. Portions in Iron and Steel. By J. E. Stead. 287 



there will be seen white lines which may or may not he indepen- 

 dent of the ferrite and pearlite areas (pi. VI. fig. 10). These white 

 lines contain the higher proportion of phosphorus. Relatively they 

 resist the corrosive action of the iodine. That this is so may be 

 verified by a longer action followed by slight re-polishing on wet 

 parchment, when, even with the aid of a simple lens, the resist 

 lines will be seen to stand in relief. When examined under oblique- 

 light rays, the phosphorised parts appear black on a light ground 

 (pi. VI. fig. 8). 



Picric Acid Etching and Tinting Method. 



The long-continued action of a 2 p.c. solution of picric acid in 

 water containing 5 p.c. alcohol will colour the portions higher 

 in phosphorus, yellow, brown, blue, etc. 



This method is well adapted for the study of wrought iron and 

 soft steel. 



When applying the reagent the specimens are immersed in the 

 solution. 



The colouring may take several minutes to develop. When it 

 is considered advisable to remove the specimens, they must be- 

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

 on no account must they be wiped with a cloth, for the slightest 

 friction is liable to remove some of the films (pi. VI. fig. 7). 



A simple solution of picric acid in water colours the phosphorised 

 portions in advance of the parts containing less phosphorus, but all 

 parts will eventually become brown if the action is continued long: 

 enough. 



v o* 



Nitric Acid Etching and Tinting Method. 



This method is based on the observation that very dilute nitric 

 acid, like iodine, acts relatively less rapidly on the phosphorised 

 portions, and at first they remain bright, but, if the action is. 

 continued, they become darkened by the formation of a dark 

 coloured skin or film. This film is probably of the same sub- 

 stance as the black residue which remains when phosphorised 

 steels are dissolved in dilute sulphuric acid. 



On etching longitudinal sections of steel and iron, the phos- 

 phorised lines at first resist the acid and appear white on a dark 

 ground, but after longer action the white lines become relatively 

 darker than the less phosphorised parts— indeed, it is possible with 

 care to obtain a positive and negative appearance on the same 

 specimen by a short or more prolonged etching (pi. VI. fig. 9). 



After strong etching, if the specimen is re-polished on a cloth 

 block, the phosphorus lines will stand in relief, and as the dark 



