The Tracing of Underground Waters. 49 



Fluorescein complies in a high degree with the requirements 

 necessary in tracing underground waters. When fluorescein is dissolved 

 in water — with the aid of a small quantity of alkali — the solution 

 possesses a brilliant green fluorescence, w'hich is destroyed by 

 acids. The fluorescent solution is reddish by transmitted light. 

 To the unaided eye the limit of visibility of the fluorescence is 

 usually about one part in forty million parts of water. This must 

 be taken only as an average, for with some specimens the fluorescence 

 can be seen when an even greater quantitv of water is used, but 

 with other specimens the fluorescence would disappear to the unaided 

 eye before quite such a high dilution is reached. 



By the use of the Marboutin fluoroscope {Comptes rend., 1901, 

 vol. 132, 365), the fluorescence can be detected when the solution 

 contains only one part in 10,000,000,000 of clear water. The 

 Marboutin fluoroscope consists of a tube of pure white glass, 95 cm. 

 long and about 1.5 cm. in diameter, which is closed at one end by a 

 rubber stopper blackened with powdered plumbago. The solution to 

 be examined is filled into this tube, which is placed in a vertical 

 position ; on looking down the tube, the fluorescein is recognised bv 

 the projection of a greenish reflection on the black stopper. Some 

 experience is necessary to avoid confusion of the natural tint of the 

 water with the green colour due to the presence of fluorescein, and 

 unless the glass is well chosen, difficulty in recognising the proper 

 green reflection is sure to arise. 



Knipscher {Pharrn. Weekbl., 1906, vol. 42, 1,042) has success- 

 fully used a method of concentrating the fluorescein by absorption in 

 animal charcoal, from which it can be dissolved out with a small 

 amount of solution of an alkali. I have found this to be quite 

 practicable inasmuch as by agitating about a litre of water, known 

 fluorescence, with about two grams of animal charcoal (of good 

 to contain fluorescein, but which to the unaided eye showed no 

 absorptive power), and then treating the charcoal w^ith 10 cc. of a 

 1% solution of caustic soda, a solution was obtained, which distinctly 

 showed fluorescence. The method is, however, somewhat cumbersome. 



I have found that the delicacy of the recognition of fluorescein 

 can be increased almost at will by the simple process of evaporation. 

 It is clear, that if a litre of water containing fluorescein be concen- 

 trated bv evaporation down to 10 cc, the intensity of the fluorescence 

 will be increased approximatelv 100 times, provided that no change 

 takes place in the fluorescein. I have tried with several natural 

 waters of different compositions, by taking 10 cc. of a solution of 

 fluorescein, in which the fluorescence was just detectable, and diluting 

 to two litres with such waters, then evaporating to about 10 cc, and 

 in no case have I failed to obtain a solution which showed fluores- 

 cence. In practice I have found it convenient to take two litres of 

 water, and concentrate to 5 cc. By this means, the delicacy of the 

 recognition is increased 400 times, and thus, if two litres of water 

 are available, fluorescein can be detected, if present, to the extent of 

 I part in 16,000,000,000; i.e., if there is only .000125 "^g- 



