616 PHEs^IDEXT S ADDRESS- — .SECTION H. 



of a luicroseopic examination, most ably carried out, of the be- 

 haviour of metals when under stress. For this purpose polished 

 surfaces were examined under the microscope, and gradually ex- 

 tende<l till they broke, the same group of crystalline grains being 

 kept under observation from the iirst application of stress until the 

 specimen was fractured. Under such conditions, up to the elastic 

 limit, the microscope shows no visible effect, but, as soon as the 

 yield point is reached and plastic deformation takes place, a remark- 

 able change occurs in tlie appearance of the crystalline grains. Under 

 a vertical illumination each. grain begins to show a development of 

 line parallel black lines, which increase in number as the strain pro- 

 gresses. Throughout any one grain these lines are approximately 

 straight and parallel, but the direction of the lines is different in the 

 different grains. The authors showed, and the point has been more 

 fully demonstrated by Mr. Rosenhaim in later work, that these 

 lines are not cracks but the edges of slips along gliding or cleavage 

 planes in the crystal, each grain apparently deforming under stress 

 in much the same way a.s a pack of cards, each card sliding a little 

 on the one below it, when pressed to one side. If strained iron be 

 heated to boiling point or allowed an interval of rest, we know that 

 it recovers its original elasticity, but, after such treatment, the dark 

 lines do not disappear from the specimen. On the other hand, after 

 repolishing the lines disappear, and cannot be brought up again by 

 etching. This goes to show, what has been still more clearly proved 

 by Mr. Rosenhaim by polishing cross sections at right angles to the 

 first, that these lines are the edges of slips that have taken place 

 along parallel surfaces in the crystal, but that the cohesion between 

 the surfaces of slip can be i-e-established, and they are not at all 

 of the nature of cracks. After severe straining a second system of 

 bands appears on some of the grains crossing the first system at an 

 angle, and, in some cases, showing little steps where the lines cross. 

 These are due to slip occm-rinc along a second set of gliding planes. 

 Such bands are developed by compression as well as by tension, and 

 the meclianism by which the grains deform in the two cases appears 

 precisely the same. Thus, when a rod of steel is strained by tension 

 or compression until it begins to give and behaves almost like a 

 viscous fluid, what happens is that the various grains of which it is 

 built np deform by slipping in tliiii layers. Each grain is like a 

 pack of cards, but the cards in one grain are not parallel to the 

 cards in the next grain. The analogy, however, is not a complete 

 one, because there is more than one set of slipping surfaces in each 

 g'rain. There is also another way in which the grain can deform, a 

 method that is more common in some metals than in others, and 

 that is by the production of twin crystals. It is possible to show 

 this phenomena on a ciystal of calcite by pressing upon it with the 

 edge of a pocket knife. Investigations have been made to ascertain 

 whether there is any distortion of the layers between the surfaces of 

 slip, and, though the point is a difficult one to settle, so far the 

 evidence is on the negative side, and the whole of the change of 

 shape suffered by the grain appears to be due to slipping in layers 

 in the way described. When the straining of a metal is carried 

 sufficiently far frlicture ultimately results, and this may take place 

 in one of two ways. Either the a'raiiis tliemselves mav suffer 



