The Presidents Address. By G. S. Woodhead. 2X9 



this, and in this I agree, except in so far that here we have a 

 practical method of sterilizing water and determining within 

 twenty minutes whether that water is sterile or not, using the 

 term " sterile " in the sense that indicates that we have cut out 

 the organisms that produce disease, i.e. that it is innocuous. By a 

 simple chemical test it is possible for an advance water -party to 

 decide how much chloride of lime must be added to 100 or 120 

 gallons of water (the usual capacity of an Army water-cart) 'to 

 render it innocuous, and then to carry out the treatment so that 

 there may be an ample supply of potable water on the arrival of 

 the troops in camp. All this can be done with no more com- 

 plicate 1 apparatus than half a dozen reputed pint mugs, a couple 

 of grams of chloride of lime, some boiled starch (or, as was pointed 

 out by my assistant, Mr. Mitchell, to a sanitary officer of high 

 rank who objected that starch was not always available, a bit 

 of boiled potato, a scrap of biscuit, or a little boiled flour, one 

 of which is always at hand), a few crystals of iodide of 

 potassium, which can be carried in a well-corked bottle in a vest 

 pocket, and a lead pencil. I will not trouble you with all 

 the calculations, gentlemen, but the following is the basis on 

 which we work. Two grams of chloride of lime (33 p.c. available 

 chlorine) contains sufficient of the chlorine-oxygen compound to 

 provide rather more than one part of chlorine per million for 

 120 gallons of water. A reputed pint mug filled to within a 

 quarter of an inch from the top contains 18 oz. or 500 c.cms., and 

 five drops of water dropped from the unsharpened end of a lead 

 pencil weigh ' 3 grm. =0*3 c.cm. If the test be carried out, as 

 it easily may with this apparatus, and it is found that with one- 

 half part of chlorine added to one million parts of water no blue or 

 purple reaction is obtained on the addition of a crystal of iodide 

 of potassium and sufficient boiled or soluble starch, then it may 

 be accepted that there is not enough of the oxy.-chlorine compound 

 present to ensure the destruction of all the above-mentioned 

 organisms present. Even if one or two parts per million be 

 added and no blue colour is obtained, it may again be accepted 

 that a large enough quantity of the sterilizing reagent has not 

 been added, and herein lies the importance of the test. It is only 

 when a " blue" reaction is obtained at the end of twenty minutes that 

 " sterilization " has been effected, and whether the faint purple or 

 blue reaction be obtained with half, one, one-and-a-half, or two parts 

 of chlorine per million of water, that indicates the amount of the 

 oxy-chlorine compound to be added for the sterilization to be com- 

 plete. If anything more than this faint tinge (i.e. a deeper blue 

 colour) appears at the end of twenty minutes, then too much of 

 the chloride of lime has been added, the water will have an un- 

 pleasant taste, and certainly will not quench the thirst as does a 

 properly sterilized water. This question of taste is, of course, of 



