386 



ELEMENTARY CHEMICAL MICROSCOPY 



the opening that is recorded, the size of the wire greatly influ- 

 ences the screened product. 



Although for industrial purposes abrasive wheels may be 

 said to conform closely to the grade and grain indicated by the 

 manufacturer, it will be found that in preparing specimens for 

 microscopic study, wheels are not easily duplicated and if we 

 purchase a wheel to replace one accidentally ruined we are apt 

 to find that it will not do just the work of the one lost. 



Wheels of softer grade and coarser grain (at high speeds) can 

 be used for roughing chilled iron and steels, hard and of high 

 tensile strength, than for material like brass soft and of 

 low tensile strength. 



No single type of wheel as to grade and grain will answer for 

 all purposes. A laboratory in which a great variety of work is 

 to be done will therefore require a series of wheels. 



TABLE VI. 

 CHARACTER OF ABRASIVE WHEEL REQUIRED. 



A fairly satisfactory system of study with reference to the 

 selection of wheels for different materials and the proper speeds 

 for grinding consists in examining with the microscope the 

 roughed surface of the specimen as ground under different con- 

 ditions and also the dust or particles falling from the wheel. 

 These particles consist of material torn off the specimen and 

 particles of abrasive and binder. The character of the dust and 

 the furrows upon the specimen will, with a little experience, 



