IOWA ACADEMY OF SCIENCES. 69 



been previously stated is a process which is in a very 

 unsatisfactory state at present. In England we find the 

 modification of the Miller-Tidy method used at present. 

 This method as modified by the Society of Public Analysts 

 we have designated as the English process. The Kubel 

 process and its modifications we find used in this country 

 and in Europe under its proper name and with slight 

 changes under other terms, such as "boiling method." The 

 time of boiling may vary from five to thirty minutes while 

 the time recommended by the American Association is ten 

 minutes. The objection which has been made against the 

 Kubel method is that at the boiling temperature the per- 

 manganate acts upon the chlorides present in the water 

 and for this reason many prefer the English method where 

 the temperature of the reaction is 80 degrees Fahrenheit. 

 The object in making the tests at fifteen miuates and four 

 hours is that the fifteen minute test indicates the amount 

 of organic matter readily putrefying and rapidly decom- 

 posing permanganate with acid. Angus Smith classed 

 this as organic matter readily decomposed and probably 

 ready to become putrid. The fifteen minute test also 

 includes in the result the action of any nitrites, ferrous 

 iron or hydrogen sulphide which may be presont. 



The object of the four hour test is supposed to indicate 

 the organic matter capable of putrefying although slow to 

 be decomposed. The total result includes the readily 

 decomposed matter in the fifteen minute test which must 

 be subtracted from the total if the amount of oxygen 

 necessary for the organic matter which is slow to be 

 decomposed is desired. The three minute test is also of 

 value in many determinations as well as the association 

 method. The association method giving results which 

 indicate the total organic matter present is much better 

 than the four hour test in many investigations, although 

 care must be taken regarding the presence of large quan- 

 tities of chlorine. 



Tidy's classification of waters based upon the oxygen 

 absorption is as follows : 



