58 MODERN SCIENCE READER 



its silvery-white and mirror-like face. Like the ordinary 

 workingman with his overalls off, a clean shave, and his 

 best clothes on, you would hardly recognize the plain 

 ordinary workaday steel rail with its old coat of brown or 

 black. 



But some one may ask, "Why do we seldom see steel 

 with this silver- white color?" We do occasionally, as in 

 razors, knives, etc., but ordinarily the moisture in the air 

 so quickly attacks such a polished surface that if unpro- 

 tected it speedily gains a coat of iron oxide or rust. A 

 metallurgist keeps all his polished specimens of iron or 

 steel in a desiccator, a receptacle which is kept free from 

 moisture by some chemical that absorbs water, and in this 

 way the prepared faces keep bright and untarnished for 

 months and sometimes years. 



Before the highly polished steel is ready to be photo- 

 graphed, it must be treated so as to make visible under the 

 microscope just what its interior architecture really is. 



The treatment consists simply in subjecting our piece of 

 rail to the action of some reagent, some chemical compound, 

 or to any treatment which will attack the different com- 

 ponents of our metal to a different degree, and which will 

 make each one stand out plainly from his fellows. Such 

 a treatment is generally an "etching," and in the case of 

 our steel rail we immerse it for a few seconds in a solution 

 of nitric acid and alcohol. The acid first attacks the junc- 

 tions of the different grains in the metal, then the grains 

 themselves, coloring some brown or black, but leaving others 

 white. Then our rail stands out in its true colors; it has 

 lost some of its previous polish, but its whole true frame- 

 work, its structure, lies plainly before us. But to the 

 ordinary observer the polished surface of the metal shows 

 practically no change; its polish is a little less brilliant, 

 and only a slight grayish appearance is visible to the 

 unaided eye. 



"How then," says the layman, "can you tell if that was 

 a good or a bad rail?" Then comes the microscope, that 



