FIELD TESTS CONCENTRATION 509 



nately the materials for three of the most important tests can be carried 



literally in a vest pocket. 



The rest of this paper consists of hints as to this field testing. 



Field Tests of Mine Waters 

 concentration 



Determination of total solids. — The first thing is to get an idea of the 

 total solids. In detailed mapping of a mine it may be enough for many 

 samples, and it saves time and chemicals if further tests are to be made 

 to know this. Leighton omits to consider it, because presumably he has 

 surface waters with specific gravity < 1.001 and less than a thousand 

 parts per million of solids in mind; Init in mine and the deeper under- 

 ground waters this test should come first. 



Specific gravity. — One index to concentration is the specific gravity. 

 To determine this the physician's urinometer is convenient and cheap 

 (50 to 75 cents). The stem is usually graduated from 1.000 to 1.060 or 

 1.080, and this is generally enough, and one can get really vest-pocket 

 sizes. They are most nearly accurate for a given temperature 60° to 70° 

 Fahrenheit. If the water is so salt as to have a greater specific weight 

 than 1.060, it may be diluted with three parts of fresh water with specific 

 gravity of 1.000. Very roughly speaking, the excess of the specific grav- 

 ity above unity in thousandths is eight times the per cent of salts. 



Electrical resistance. — Schlichter and the U. S. Department of Agricul- 

 ture^^ have measured the amount of salts by the electrical resistance. 



I have no practical experience with the method. It appears to be much 

 more delicate than the others and practically the only method giving quan- 

 titative results with weak concentrations. For how small quantities and 

 how light the apparatus could be made remains to be seen. 



Total rrflectometer. — The urinometer requires at least an ounce or so of 

 water and is slightly fragile. To test the concentration of a drop or two, 

 the variation in the angle of total reflection seems the most practical 

 method. Bausch and Lomb have made for me, with mutual interchange 

 of suggestions, a pocket total reflectometer, which is figured herewith 

 (figure 1) and gives 70 divisions on the scale between fresh water and 

 salt. It is thus about one-third as sensitive as the urinometer. Of course, 

 the index of refraction is not exactly proportional to the density. 



The method of use is as follows : A little of the water to be tested is 

 placed in the triangular prismatic trough A, one side of which is a glass 

 prism, B. Light entering the prism below, through a window at C, is 



^ See the Bureau of Soils field soil book and the methods and references there given. 

 XLVII — Bull. Geol. Soc. Am,. Vol. 19, 1907 



