INTRODUCTION 



of gravel soil ; further, that the supply of a tube-well became 

 contaminated by B. prodigiosus when cultures of the latter were 

 inserted into the surface 14 feet from the top of the well.^ 

 Characteristic bacilli, like prodigiosus and violaceus, have fre- 

 quently been used with success for testing filters and leakages. 

 Where the suspected source is accessible, a quicker method is 

 to add a.quantity of some easily recognisable substance, either 

 in solution, such as fluorescein, or in suspension, such as solid 

 starch, and to look for it in the water affected. The presence 

 of sewage will also reveal itself in the analysis. I will give an 

 example that has lately come within my own experience of an 

 infiltration that passed through a distance of about half a mile. 

 A public school on a hill in the country was supplied with a 

 well-water (A), while its sewage was treated on a farm below. 

 Near the lower end of the farm was a well (B), which served as 

 a portion of the town supply. It was sunk in the Hythe beds 

 of the lower greensand, and the ground-water flow was from 

 the hill to the valley. My analyses were as follows : 



II. 



proving distinctly the access of pollution from the sewage farm. 



A somewhat similar case in December, 1901, led me from the 

 chemical results to undertake a special and detailed search for 

 the typhoid organism in the water of a shallow well in Surrey, 

 and I succeeded in isolating typhoid bacilli.^ 



3. Means of access must be provided for the scavenger, so 

 that the filth need not be carried through a dwelling. 



1 Zeits.f. Hyg., 1897, p. 549. '^ Analyst, August, 1902. 



