CORRESPONDING SOCIETIES. 431 



Advisory Committee. The actual work of taking observations at the different 

 stations lias been done by or under the local sanitary authorities. The Committee 

 have now about twenty-five stations scattered over different parts of the country, 

 from each of which monthly reports are received upon the quantity of impurity 

 deposited in a standard gauge. Blank reports are sent to each station for filling 

 ill, and these are returned monthly to the Committee's office for filing. 

 There have been three main lines of investigation followetl : — 



(1) The measurement of deposited impurity. 



(2) The measurement of suspended impurity. 



(3) The measurement of acidity. 



Most of the data accumulated by the Committee up to the present have been 

 on deposited impurity. The investigation of suspended impurity and acidity 

 of the air is not in such an advanced state, and the Committee are at pi'esent at 

 work upon the problem of the best methods of investiajation to adopt. 



Deposited Im-purity. 



The special standard gauge devised by the Committee for the measurement 

 of deposited impurity consists of a circular gauge vessel of cast iron, coated 

 with a vitreous enamel, supported in a suitable stand. This vessel exposes a 

 catchment area of about four square feet, and receives the total deposit from 

 the air, which, together with the rain, is passed into bottles fixed underneath the 

 gauge vessel. 



At the end of each month the gauge vessel is washed down with some of 

 the water collected, and the water and deposit collected in the bottles during 

 the month is removed for analysis. The insoluble matter is filtered off or 

 allowed to deposit, and the soluble matter estimated separately. A complete 

 analysis of the deposit is not aimed at, but it is divided into the following 

 constituents : — ■ 



Rainfall ; insoluble matter, which is sub-divided into tarry matter soluble 

 in CS2, carbonaceous matter other than tar, and ash; soluble matter, which is 

 sub-divided into loss on ignition, and ash. The sum of the soluble and insoluble 

 being returned as ' total solids.' 



Although a complete analysis is not made of the soluble matter, certain of 

 its constituents are further estimated. These are sulphates as SO3, chlorine as 

 CI, and ammonia as NH3. 



All the constituents of the deposited matter are returned as shown in the 

 blank report form herewith. The volume of water is given in litres and mm. 

 of depth. Other constituents are shown in grammes, percentage of total solids, 

 and in metric tons per square kilometre, the latter form being chosen in view 

 of the Committee's hope that tlie investigation will become international. 



In the following table the composition of the deposit at four representative 

 stations is given. 



Thifi table speaks for itself, and shows the great variation in the quantity and 

 composition of the deposit at different places. 



A second table is included which shows for the ^ast five years the total im- 

 purity, deposited from the air at different stations ; the figures give mean monthly 

 deposits in metric tons per square kilometre. This table shows only the total 

 impurity, as this will be sufficient for the present purpose. 



The division of the year into summer and winter has been adopted in all the 

 Committee's Reports, as it was felt that this was a somewhat natural division 

 which should be followed. 



Referring to Table II, we may compare the deposit at the diiTerent stations 

 with each other, and we may examine the figures for any variation from y^ear to 

 year. 



Although the War has interfered with the continuity of the observation.<! at 

 many of the stations, and the blanks in Table II. are caused bv the suspension 

 of observation at different places, the Committee has been fortunate in being 

 able to carry on work diiring the whole period at about 2.5 stations. Observa- 

 tions will be renewed shortly at some at leost of the places which suspended 

 work. 



