367 



and articles on which Mr. Hiorns has based his book are 

 scattered and highly specialised, and the acquirement of a 

 general elementary knowledge of the subject has been a difficult 

 matter for the student. Teachers have been seriously handi- 

 capped in their efforts, and last, but not least, original 

 investigators have had no book of reference to aid them in 

 their researches. 



The appearance of this work is therefore particularly well 

 timed, and it will be eagerly examined by many who will be 

 anxious to know how far it will suit them. It may be said at 

 once that they will have no right to be disappointed by what 

 they find. 



Mr. Hiorns has collected a large number of the more im- 

 portant results which have been obtained recently, and has given 

 a capital summary of them which is surprisingly complete con- 

 sidering the moderate size of the book. The introduction, which 

 contains some account of the history of the subject and some 

 general considerations upon it, is perhaps the least satisfactory 

 chapter. The working directions, however, for preparing pieces 

 of metal for examination and for photographing them, are 

 sufficient for the purpose. 



The greater part of the book is devoted to the metals which 

 are most important in industrial work, that is, to iron and steel 

 in the first place, and to the alloys of copper in the second. It 

 has long been known that the appearance of the fractured 

 surface of steel may be misleading and give little clue to the 

 nature of the specimen. It is necessary to make a clean cut 

 with a saw through the metal, to polish the surface with several 

 emery papers in succession, each one of finer grain than the 

 last, and to finish with the finest rouge spread on broadcloth and 

 wetted thoroughly. The polished surface is then etched by acids 

 or by rubbing with a solution of ammonium nitrate or Uquorice, 

 and mounted for examination with the microscope. 



It can then be seen that metals generally consist of large, 

 irregular crystalline grains, each composed of a large number of 

 secondary crystals arranged with some regularity. The primary 

 grains vary enormously in size, and may be as much as an inch 

 in diameter. The secondary crystals in steel, however, are ex- 

 ceedingly minute, and may require a magnification of a thousand 

 diameters to be resolved. As the examination must be by re. 



