510 



A. C. LANE MINE WATERS AND THEIR FIELD ASSAY 



totally reflected up to an angle dependent on the relative indices of A and 

 B. Tims an objective, D, has part of the field more illuminated than the 

 rest. These parts are separated in daylight by a horizontal colored band. 

 The mine lights are generally more nearly monochromatic. It is perhaps 

 well, by cementing a bit of yellow glass or celluloid, to have the light at C 

 nearly monochromatic. The image of the field is found at E, and there 

 a scale is placed which is viewed by the ocular F. Light should be cut off 

 from entering except by C, by shading by the hand or otherwise. Down 

 in the mine, the sole illumination being from a lamp or candle held a little 

 below C, this causes no difficulty. 



Figure 1. — Outline of Pocket Total Seflectomefer 



Outlines of Instrument In full line. The course of a ray from C through the glasa 

 prism, reflected at the angle of total reflection, reading the scale E, Is In dots. The part 

 of the scale above will be more illuminated. 



TABLET METHODS 



U. S. Geological Survey method. — In following the example of the 

 physicians and having reagents put up with known quantities in tablet 

 form, the U. S. Geological Survey-^ has performed a real service, which 

 enables one to carry in a pocket the chemicals to make three or four im- 

 portant tests. It seems to me this method is capable of much greater 

 development. It must also be remembered that by dissolving a tablet in 

 a known quantity of water a solution of known strength may be obtained, 

 and this is often the most convenient method, if a numl3er of tests are to 

 be made at a time. The accuracy of most of these methods is dependent 

 upon the accuracy with which a quantity of water varying from 20 to 100 

 cubic centimeters can be measured, and is something like 1 to 5 per cent. 



Chlorine. — Leighton's method seems all that could be desired in deli- 

 cacy, accurac}', simplicity, and convenience. It is fortunate that what in 

 my work appears the most vital determination is so convenient. The 

 potassium dichromate indicator can readily be carried solid. Less than a 

 cubic millimeter need be dissolved. 



In the case of waters likely to be strong, it will be well to add the potas- 

 sium dichromate and a tablet or more of silver nitrate to distilled water, 

 or a water whose chlorine has been determined, then from a burette drop 



^ Leighton : Water supply paper no. 151, p. 51. 



