Sterilisation Experiments on the Cambridge Water. 571 



each, taken during this period, every one was sterile, no cohform 

 organisms being found in 6^ litres of the treated water. More- 

 over on no single occasion was there either taste or smell of 

 chlorine ; the water was clear and bright, and fresh and palatable. 



From these experiments I am satisfied (a) that sterilisation 

 of the Cambridge water by bleaching powder is not only efficient 

 but is easily carried out ; for given the faintest trace of chlorine 

 in the treated water as it comes from the chlorinating cylinder at 

 the end of a somewhat indefinite period of treatment (more than 

 20 minutes) sterilisation was complete ; (6) that in the case of the 

 Cambridge water it is unnecessary to add bisulphite of soda, the 

 process thus being enormously simplified ; (c) that the trace of 

 chlorine remaining at the end of treatment disappears very rapidly 

 as the water passes through the pipes or as it is exposed in the 

 reservoir. 



The amount of chlorine remaining at the end of treatment 

 may be readily measured by any intelligent labourer supplied 

 with a bottle of iodide of potassium crystals and a flask of filtered 

 starch ; a crystal of iodide of potassium, a few drops of acetic acid 

 and a tablespoonful of starch solution added to a litre of the water 

 in a glass jug held over a white tile or a sheet of white paper 

 enabling the observer to determine at once whether it has a blue 

 or a violet tint or is clear. If there is a deep blue tint there is 

 too much chlorine present ; a violet tint is the proper " end re- 

 action " showing the presence of a faint trace of chlorine, whilst, 

 if there is no colour at all the amount of chlorine present is 

 not sufficient to ensure sterilisation. This work was carried out 

 mainly by my assistant, Mr Mitchell, to whom I am greatly 

 indebted for the careful manner in which these observations 

 were made. He remained at Fulbourn during the whole time 

 over which the experiments extended, but after a time the 

 engineer who looked after the pumps was detailed to make 

 these colour estimations. These I compared with my own and 

 Mr Mitchell's estimations and found that they were in all respects 

 most satisfactory. This, of course, is an exceedingly important 

 practical detail for it is evident that the chlorine may be made to 

 serve as an indicator of the presence of organic matter in water. 

 Instead of waiting for a bacteriological examination, the simplest 

 and shortest of which takes several days to carry out, an estimate 

 of the amount of chlorine absorbed may, if necessary, be made from 

 hour to hour, the amount of chlorine required to give the violet 

 reaction at the end of contact being the amount necessary to 

 ensure complete sterilisation. 



From these experiments then I am satisfied that from the 

 point of view of ease of application, certainty of action, and 

 absence of interference with the physical characters of the water. 



