PRESIDENTIAL ADDRESS. 717 



them for the welfare of the whole organism. The plastic nature of nervous 

 tissue renders it, in accordance with the principles of natural selection, par- 

 ticularly favourable for progressive change in this direction, and thus developments 

 may occur which reach their highest physiological expression in the brain of 

 man. 



In conclusion attention may be drawn to the peculiar instability of living 

 processes and structures. The living units show that significant mutability which 

 the physiologist describes as metabolism. This mutability appears to be encouraged 

 or discouraged by the extent to which it fulfils a purpose, and this purpose in a 

 living organism is the dominating law of its own development. The fulfilment 

 of this purpose by means of physical and chemical change is such a general 

 characteristic of living processes that a physiologist may with some confidence 

 suggest that this fulfilment is the distinctive mark of a living thing. 



The following Paper and Report were read : — 



1. Granular Changes in Nerve Fibres. 

 By Professor J. S. Macdonald, B.A. 



2. Report on the Metabolic Balance Sheet of the Individual Tissues. 



See Reports, p. 426. 



FRIDAY, AUGUST 3. 



The following Papers and Reports were read : — 



1. The Nitrification of Sewage in Shallow Filters with Fine Particles. 



By Dr. George Reid. 



The author has always advocated fine-grain sewage filters, used as ' perco- 

 lating ' filters, not as ' contact ' beds ; but until recently he was not aware that 

 the reduction in the size of the filter particles allowed of much shallower and, 

 therefore, less costly filters being constructed. 



At present the Local Government Board pay Jio regard to size of particles or 

 depth of filter so long as a minimum depth of four feet is provided, the sole govern- 

 ing principle being cubic capacity in relation to sewage flow, irrespective even of 

 the strength of the sewage. It follows, therefore, that for a given volume of 

 sewage the area of an 8-foot filter need only be half that of one four feet deep, the 

 rate of delivery per superficial yard in the case of the former being twice that of 

 the latter. 



Seeing that nitrification is dependent upon the healthy activity of aerobic 

 organisms, theoretically it would seem that, the larger the number of these organisms, 

 the greater the amount of work they can accomplish. If this is so, the aim surely 

 should be to provide, within a given space, as large a surface for bacterial growth 

 as possible, by reducing the filter particles to the smallest size which is found to 

 be compatible with free aeration and practical working conditions. 



In practice, the author has found that the best results are obtained from 

 |-inch filter particles, although from 1 to 3 or 4 inches is the range usually 

 adopted for percolating filters. 



The author had recently an exceptional opportunity of following up the pro- 

 cess more in detail at disposal works which had been in constant operation 

 for nearly four years, yielding invariably high-class eflluents. Accordingly, the 



