The Micro- Examination of Metals. 
7 
appearance, and, whilst the work is by no means complete, figs. 7 
and 8 show a striking similarity to the corrugations produced on the 
surface by platinum and palladium. These micro-photographs 
have been obtained from the polished and etched sections of silver 
beads containing 1*4 p.c. and 3 p.c. of platinum and palladium 
respectively. 
The advantages of the method as described are fairly obvious, 
for no preliminary polishing, etching or other preparation of the 
surface is necessary, and either oblique or vertical illumination 
may be used. 
3. The Constitution of the Alloy. 
An alloy may consist of a single constituent, or it may be built 
up of two or more constituents, and by means of microscopic 
examination, preceded by suitable etching, differentiation of these 
constituents can be made. 
Manganese bronzes may be cited as illustrations of this point. 
These so-called bronzes are copper-zinc alloys with small percen- 
tages of other metals, such as tin, manganese and iron, and there- 
fore come under the category of brasses. Such alloys are used for 
a variety of purposes in the engineering industry, and in particular 
for marine propellers. The composition of these industrial brasses 
can be estimated satisfactorily by micro-examination in conjunc- 
tion with the equilibrium diagram of the series. The three phases 
are named respectively alpha, beta, and alpha + beta. Cast 
brasses in the normal condition, which show the alpha structure, 
contain the higher percentage of copper and are the weaker alloys, 
whilst those with an all-beta structure are poorer in copper, but 
are superior as regards tensile strength. Alloys with the alpha + 
beta structure represent intermediate compositions with a tenacity 
between the two outside limits. 
Figs. 9, 10 and 11 show the structure of two manganese bronzes 
with 75 p.c., 40 p.c. and 5 p.c. alpha respectively. 
4. The Distribution of the Constituents. 
In an alloy composed of more than one constituent it is 
important that the various constituents shall be distributed evenly 
throughout the mass if maximum strength and uniformity of 
composition and physical properties are to be attained. Owing to 
the different rates of cooling of a metallic mass from the completely 
liquid to the completely solid state, and the difference in freezing 
point of the constituents of the alloy, it is not an uncommon thing 
