i6o 



ELEMENTARY BIOLOGY 



In polishing furniture powdered pumice stone and oil can be used 

 instead of sandpaper. 



In some dusty processes it has been possible to inclose the 

 machinery and the material in such a way as to prevent the escape 

 of dust into the air breathed by the workers, as, for example, in flour 

 mills, in certain operations that have to do with the crushing of 

 ores and minerals, and in the polishing of small metal objects. 



Fig. 53. Dust and safety hood 



In polishing metal goods this hood protects the worker from dust and, in case the wheel 

 bursts, from flying particles. (From photograph by New Jersey Department of Labor) 



But where these methods are not possible, it is necessary to 

 use a special form of ventilation that draws the dust away from 

 the point at which it is generated. Special hoods connected 

 with exhaust pipes are thus placed over grinding wheels, over 

 rotary saw-blades, over polishing wheels, and so on (Fig. 53). 



Even with all the precautions mentioned, there will be dust 

 in many workrooms. In such cases the individual worker 

 should carry an air-filter over his mouth and nose. This 



