The Determination of Organic Matter in Air, 239 



These methods are open to one or more of the following objec- 

 tions : — 



1. The time required for a single determination is very considerable, 

 and recessarily varies with the amount of organic matter present. 



2. There is great uncertainty as to whether the organic matter has 

 been fully absorbed and acted on by the permanganate in the first 

 method, or absorbed by the water in the second. 



3. It is difficult to determine the exact point of full bleaching, or 

 to estimate by oxalic acid very small degrees of partial bleaching. 



4. The methods are inapplicable (chiefly on the score of time and of 

 the extent and complication of the apparatus required) in circum- 

 stances and places where such determinations are most desirable. 

 Analyses, especially by the second method, cannot be completed on 

 the spot, except in very rare cases, so that unless a large quantity of 

 apparatus be taken, several consecutive determinations in a series of 

 buildings or rooms is impossible (cf. De Chaumont, 'Roy. Soc. Proc.,' 

 vol. 23, 1875). 



5. The results obtained by the method proposed below show that 

 very considerable variations in the organic matter are sometimes 

 liable to occur within the period of determination required by 

 Dr. Angus Smith's method. 



By the new process, which it is the object of the present paper to 

 describe, the above difficulties are in a great measure overcome. The 

 special advantages we claim for it are : — (1.) Rapidity and simplicity 

 of execution. (2.) A higher probability, though not absolute cer- 

 tainty, that the organic matter is fully absorbed. (3.) A more general 

 applicability. 



The principle of the process is the same as that of Angus Smith's 

 first method, viz., reduction of potassium permanganate. It differs, 

 however, from Smith's method, more particularly in the mode of 

 estimating the amount of reduction. This consists in determining 

 colorimetrically , by comparison with a standard, the fractional bleach- 

 ing effected in a given volume of air. 



Method. — The solution of permanganate used is of strength, of 



which 1 c.c. == O008 mgrm. of oxygen = 0*0000056 litre of oxygen at 



0° and 760 mm. It is usually kept of — strength, and dilated as 



required, about 50 c.c. of dilute sulphuric acid (1 to 6) being added 

 to each litre of the weak solution. 



For the collection of the samples of air large well-stoppered jars of 

 about 3"5 litres capacity are used. The jars are first rinsed out with 

 a little standard permanganate, and when not in use some of the solu- 

 tion is always left in them, so as to ensure complete cleanliness from 

 any reducing substance. Before use the jars are drained, and the 



