50 Report S.A.A. Advancement of Science. 



present. Hypothetically, the delicacy can be increased either by 

 starting with a larger quantity or by concentrating further — but 1 do 

 not desire to attach much importance to the quantitative relationship, 

 for I find in practice that 2 litres is a sufficient quantity to take, and thai 

 it is not advisable to concentrate below 5 cc, otherwise the observation 

 becomes difficult, and if the water contains dissolved matter, which 

 may have separated on evaporation, the solid must be separated by 

 filtration , 5 cc. occupies a convenient space for observation in a tube 

 of rather less than i cm. in diameter. In order to ensure that the 

 final solution is alkaline, it is always well to add a drop of ammonia 

 solution, although this is usually unnecessary, particularly if the eva- 

 poration has taken place in a glass vessel. Should ammonia cause 

 any precipitation, the precipitate must be removed by filtration. 



I would particularly sound a note of warning that one must be 

 on guard in observing the final solution, not to mistake a greenish 

 colour, which often results on the evaporation of water, for the 

 coloration due to fluorescein. If iron is present in the water, this 

 greenish tinge is likely to be obtained. It is, therefore, advisable, 

 at any rate until some experience is gained in distinguishing the 

 colours, to compare the evaporate with water to which a minute trace 

 of fluorescein has been added. 



Absorption of Fluorescein. — The absorption of fluorescein 

 was first noticed by Trillat {Bull. Soc. beige geol. 1903, 

 17, 301). He states that a large amount of calcareous 

 matter in solution (especially the carbonates), will decolorise the 

 dye to some extent. This, however, was not the experience of 

 Dr. Copeman, who in connexion with the investigation for the 

 Local Government Board, on an outbreak of enteric fever at Ful- 

 bourn Asvlum, near Cambridge, was led to endeavour to trace the 

 contamination of a hard underground water by means of fluorescein, 

 and who passed a solution of fluorescein through columns of chalk, 

 without observing any diminution in the intensity of the colour. 

 (Report of Medical Inspectors of the L.G.B., No. 229, 1906). 



From some preliminary work it had been found that on passing 

 a I in 550,000 solution of fluorescein in water through a column of 

 soil, the fluorescein was at first completely removed, but as filtration 

 proceeded, the filtrate was found to contain fluorescein, and ulti- 

 matelv the solution passed through undiminished in fluorescing power. 

 Prolonged percolation of water showed that the fluorescein could be 

 completelv washed out of the soil. By substituting a fine-grained 

 sand for the soil, exactly the same phenomenon was observed, and 

 further, it was found that precisely the same results were obtained, 

 when use was made of a solution of fluorescein made alkaline with 

 sodium hydroxide. 



In order to ascertain if the extent of the removal of the fluor- 

 escein from solution was proportional to the amount of sand through 

 which the solution filtered, three tubes of the same internal diameter 

 (i.9C.m.) were fitted with stoppers through which passed narrow glass 

 tubes. A laver of coarse gravel— about 3 cm. deep— was placed in 



