The Micro-Examination of Metals. 
3 
kept within reasonable limits of size, the author desires to draw 
attention to the first four points in particular. 
1. The Presence of Foreign Bodies. 
The term “ mechanical enclosure ” is sometimes given to these 
foreign bodies, as they are not an integral part of the substance in 
question, and have been entangled mechanically by the metal 
during the process of production, or by the alloy whilst in the 
fused state. 
Wrought iron always contains slag patches due to the iron 
having been produced in the pasty state, a condition which is not 
ideal for the complete separation of slag from metal. These 
patches appear as extended or elongated enclosures if the section 
examined is in the direction of working, or as more or less rounded 
masses if the section taken is at right angles to the direction of 
rolling, drawing or forging. 
It might be pointed out here that the chemical composition 
of wrought iron and very low carbon steel may be almost identical, 
but the presence or absence of slag patches as revealed by the 
microscope indicates clearly to which class the metal belongs. 
Another common inclusion in iron and steel is manganese 
sulphide, which occurs in the form of pools or patches of a dove- 
grey colour, and these patches vary considerably both in shape 
and size. 
Illustrations of slag patches and manganese sulphide may be 
found in text-books on Iron and Steel, and General Metallography. 
2. The. Existence of Small Quantities of Metals, 
Metalloids, etc., which may or may not exert 
an Injurious Effect on the Metal. 
Small quantities of impurities may exert a great influence on 
the physical properties of metals and alloys, and whilst the 
existence of such impurities can be ascertained by micrographic 
examination, chemical analysis must be employed when a quanti- 
tative result is required. Phosphorus, for example, has a marked 
effect on iron and steel, small quantities causing the latter to be 
cold short and unreliable under shock, and therefore for that 
reason the maximum percentage allowed is about 0*05 p.c. 
During the war period, when so much had to be sacrificed for 
output, a large quantity of steel was turned out with a somewhat 
higher percentage of phosphorus than now quoted as the maximum, 
B 2 
