CORRESPONDING SOCIETIES. 437 



2,000 cub. ft. Hence some other method was sought for, and this took the 

 form of a small filter, consisting of a pair of aspirating bottles, by which 

 two litres of air could be drawn through a disc of white filter pai)er ^ inch in 

 diameter; a disioloured mark was left upon the disc, the depth of discoloration 

 depending upon the amount of impurity in the air. A standard filter was got 

 out on these lines, with which a record can be taken in about fifteen minutes, 

 and the quantity of impurity estimated by the aid of a scale of shades, which 

 scale has been calibrated by weighing, as described in the Committee's third 

 Report for 1916-17. 



As the nature of the impurity in cities is suflficiently alike, it was felt that 

 the records thus obtained would give a reasonably correct idea of the amount 

 of suspended impurity. 



The subsequent development of this instrument took the form of an automatic 

 filter on the following lines : — 



In this apparatus an aspirating vessel is provided, into which water is 

 admitted through a regulating cock at the bottom. A syphon is fixed inside 

 the vessel, which causes the water-level to oscillate at regular intervals between 

 two fixed levels in the vessel ; thus, while the water is rising, air is driven out 

 of the vessel, and while it is falling air is aspirated into the vessel. 



On tlie top of the instrument an entrance for air is provided of a fixed 

 diameter, over which revolves a disc of filter paper. Provision is made by 

 which this disc is held against the entrance during the time the air is being 

 drawn into the vessel. The disc is caused to revolve by a weight and string, 

 but the rate of it« revolution is regulated by a stop which follows the hour hand 

 of a small clock placed above the disc. The force for clamping the disc is 

 obtained by a pressure-operated flexible diaphragm acting through a lever. The 

 records are given upon a disc of paper upon which the hours have already been 

 marked similarly to a clock-face, and each record is placed automatically 

 opposite the time at which it is taken. Thus the interval between records is 

 unimportant, although this is easily adjustable. 



A sample of the latest form of this instrument was exhibited, and it is hoped 

 that useful information will be obtained by its use. Owing to war conditions, 

 it has not been possible to complete the automatic filter until this month ; but 

 it is now ready, and the Committee hopes that it will be put to extensive use. 



The calibration of the automatic filter is the same as the single-record instru- 

 ment, as both operate with a volume of two litres of air and filter this through 

 a g-inch disc of paper. The scale of shades enables the observer to read directly 

 the quantity of impurity in milligrams per cubic metre, as each unit of shade 

 number represents 0-32 mg. per cubic metre. 



Acidity. 



A special investigation into the acidity of the air has been undertaken, and 

 is being carried out by the Cbmmittee's Eeseareh Chemist, Mr. G. M. Watson, 

 B.Sc. The experimental work carried out is fully described in the Committee's 

 fifth Report for the year ending March 31, 1919, which will shortly be published. 



The great difficulty again has been the small quantity of acid to be measured, 

 so that the ordinary methods of chemical analysis were not directly applicable. 

 A special instrument was, however, designed in which a solution of methyl 

 orange in distilled water is used as an indicator. A few cubic centimetres of 

 this solution are placed in a specially designed absorption tube and a measured 

 quantity of air bubbled througih it. If this solution be absolutely neutral to 

 begin with, which it rarely is, and the air is acid, then the colour change in the 

 methyl orange commences at once, and it passes gradually from yellow to red as 

 the air as bubbled through it. The amount of colour change is estimated by 

 means of a tintometer tube placed beside the absorption tube, both being so 

 designed as to permit one to look through the liquid vertically from the tops of 

 the tubes. It was found that the degree of colour change was proportionate to 

 the amount of acid in the air, and that, using the same solution of methyl orange 

 in the tintometer tube, its colour could be matched to the changing liquid in 

 the ab-sorption tube by varying the thickness of the layer of methyl orange 

 .solution in the former, as it was found that as the layer looked through was 

 increased in thickness, so viewed in this way the colour changed from yellow 



1919. K R 



