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 



