PETROGRAPHY AND METALLOGRAPHY. 163 
microscope, owes its beginnings to Dr. Early of Sheffield 
and Professor Martens of Charlottenburg. Its aim is to 
study the minute structure of metals with the microscope, 
not, as in the case of minerals, by grinding off thin sections, 
but by examining polished surfaces by reflected light. 
Its importance is now so great in the testing of iron and 
steel as to demand some attention in the most elementary 
survey of the field of microscopy; and since it is with the 
various alloys that metallography is principally concerned, 
certain general characteristics of these bodies must first 
be briefly discussed. ; 
Many alloys may, for practical purposes, be considered 
as solutions of one metal in another; the alloy of silver 
and copper is a good example of this type. If a molten 
alloy containing more than 72% of silver be gradually 
cooled, a point is reached at which the rate of cooling 
becomes retarded and silver begins to separate. The 
greater the amount of silver the higher is the tempera- 
ture at which this process begins. After a time, when 
the temperature falls to 770° C., the still molten silver 
will be found to have fallen to just 72%; a second retarda- 
tion in the cooling is now apparent accompanied by the 
solidification of the entire alloy. If more than 28% of 
copper be present, on the other hand, the first retardation 
is accomplished by a deposition of copper, which con- 
tinues until the excess has all been solidified, which occurs 
at a temperature of 770°C. The remainder then solidifies 
entire. A molten alloy containing originally 72% of 
silver and 28% of copper shows only one point of retarda- 
tion, remaining molten above 770° and becoming solid 
