﻿organic matter in air. 11 



Method YL 



Bj this method the sulphuretted hydrogen and the sulphurous acid 

 gas in the air are determined directly by aspirating air through a Petten- 

 kofer absorption tube containing 100 c. c. of a -f^ solution of iodine. 

 The quantity of iodine remaining unchanged in the solution is deter- 

 mined by titrating with a yw solution of sodium hyposulphite, using 

 starch paste as an indicator. One c. c. of the sodium hyposulphite solu- 

 tion is equal to 1.7 mgs. of HgS, or 3.2 mgs. of SOg. The results 

 obtained by this method are given in Table XI. 



The results obtained by these methods show quite variable amounts 

 of organic matter in the air. This is what one would expect to find, 

 but the great difficulty in such an investigation is that it is not possible 

 to make satisfactory control experiments. There is no possibility of 

 establishing a standard for comparison, yet by making simultaneous 

 determinations with the different methods on the same au* some idea 

 as to their reliability may be obtained. The experimental error in each 

 of the methods is necessarily very large, and in some of the methods 

 is so great as to render them almost entirely useless. The more cum- 

 bersome the apparatus and the more complicated the method the greater 

 will be the experimental error. Since there is no way known as yet of 

 determining the organic matter directly, it becomes necessary to 

 select the method, which gives the most closely concordant results 

 in air of approximately the same degree of purity, or in several 

 duplicate determinations on the same air. The method which con- 

 stantly gives the lowest results in simultaneous determinations is also 

 probably the one to be preferred as being the most reliable. This 

 method will naturally be the one in which the danger of contamination 

 can be reduced to a minimum, and requires the simplest form of ap- 

 paratus, and yet affords reasonable certainty that all of the organic 

 matter is absorbed. 



Judging from the results obtained in the several series of simultane- 

 ous determinations (see Tables XII to XY), it is no easy matter to decide 

 which of the different methods is to be preferred as being the most 

 reliable. Taking all the points into consideration, however, it appears 

 to me that the Eemsen absorption tube and the absorbent material 

 recommended by Eemsen — freshly ignited granular pumice-stone — 

 affords the most trustworthy results. The absorption tube is small 

 and easily cleansed, and the pumice-stone can be freed from organic 

 matter by prolonged incineration in a platinum crucible. The pro- 

 cess of distillation employed in this and several of the other methods 

 is, however, liable to lead to considerable error, especially in inex- 

 perienced hands. The amounts of ammonia to be derived from the 

 organic matter in several hundred litres of air are extremely small, so 



