president's address SECTION H. 615 



The credit of being the tir.st to appiy the microscope to the 

 difficult study of the micro-striicture of metals belongs to Dr. Sorby, 

 oi Sheffield, who in 1864 published the first paper on the microscopic 

 structure of iron and steel. Since then metallography, as it is 

 termed, has become almost a science in itself, claiming many 

 workers, and the literature of the subject has become extensive. 

 The microscopic examination is done by first finely polishing a 

 section, lightly etching with acid, and then viewing it under the 

 microscope by means of reflected light. Most of the work has been 

 carried out from a metallurgist's standpoint, its object being to 

 deteraiine the nature and proportion of the different constituents in 

 various grades of iron, steel, and alloys, and the manner of their 

 distribution. In this way it has proved of great practical utility. 

 In one way it is unfortunate that so much of this work has been 

 done upon iron and steel, because not less than five or six ilift'erent 

 constituents are commonly recognised in its microscopic sti-ucture, 

 and the phenomena exhibited are much more complex than those 

 shown by simpler and purer metals. It is, of course, liatnral that 

 this should be the case because of the practical importance of these 

 metals, Ijut it seems probable that the purer metals may enable 

 some of the fundamental processes, such as hardening under strain, 

 to be studied more easily. One important result of these observa- 

 tions is to show that the structure of metals in general is always 

 crystalline, and that an iron bar with all its properties of ductility 

 and elasticity, is nothing but an aggregate of ciystal grains. It 

 was a common thing for engineers to speak of iron or steel, that was 

 supposed to have become brittle through being subjected to excessive 

 vibration extending over long peinods, as having become crystalline. 

 This work has taught us that the metal is always crystalline. The 

 size and character of the crystals may alter; we know that they 

 alter when subjected to heat as in the processes of annealing, and 

 possibly their nature may change under the influence of long-con- 

 tinued vibration, although there is at present no proof of this, but, 

 brittle or ductile, the bar is always built up of crystals. Iron or steel 

 in a brittle form is usually composed of larger ciystals than when 

 it is in a ductile or malleable condition, but otherwise its general 

 structure is the same. The polished and etched surface of a metal 

 under the microscope exhibits a series of grains of irregTilar shape 

 in close contact with one another. Evidence, which we need not 

 detail, shows that each one of these grains is a separate ciystal ; in 

 the process of ciystallization each crystal has apparently grown from 

 a nucleus until its growth has been stopped by contact with neigh- 

 bouring grains. As the rate of growth is not uniform in every 

 direction their resultant shape is irregular. Throughout each crystal 

 the molecular an-angement is uniform, but the direction of the mole- 

 cular arrangements in adjoining crystals is different. Each grain we 

 may picture as built up of layers of molecules, the layers being all 

 parallel throughout the grain. But the direction of these layers in 

 one grain will not be parallel to the layers in adjacent grains." From 

 an engineering point of view a memorable paper by Professor Ewing 

 and Mr. W. Rosenhaim (1851 Exhibition Research Scholar of the 

 IJnijersity of Melbourne) appears in the Transactions of the Royal 

 Society of London for 1900, in which the autJiors detail the results 



