Manchester Memoirs, Vol. H. (1907), No. 15. 9 



little geometrical pits produced in the process of etching, 

 which upon close examination turn out to be true 

 rectangular pits, with sides which have parallel directions 

 all over the crystal. 



Examine it under a high power, and you find these 

 geometrical markings parallel to one another, showing 

 that there still is a true crystalline structure within the 

 grain. How has that crystalline structure been preserved ? 

 Mr. Rosenhain and I, in the course of an investigation 

 carried out some years ago in Cambridge, found that the 

 straining of the individual grain took place much as a 

 pack of cards is strained when you make one card slip 

 over another. 



This was observed by watching an individual grain 

 under the microscope in a piece of iron. When it is 

 strained a number of markings appear on it, in the form 

 of lines, more or less straight and parallel, which appear 

 dark under the microscope. A close examination shows 

 on any severely strained crystal two, and in some cases 

 three, systems of such lines. 



Mr. Rosenhain and I called these lines "slip-lines," 

 because they are lines formed by a process of slipping. 

 They run, in a general way, parallel, and then there is 

 another system, or it may be two systems, crossing them, 

 running in other directions. As you pass from one grain 

 to another you find the grains distinguished by differences 

 in the directions of the slip lines. Fig. 9, Plate V, shows 

 slip-lines as they manifest themselves in a piece of cast 

 lead, after it has been subjected to a process of straining. 

 Over each crystal of lead no fewer than three distinct 

 and separate systems of lines can be plainly seen. 



To understand what these lines really are, consider an 

 imaginary section {Text-fig. 1) through a couple of grains 

 of metal. The line on the top represents the surface at 



