408 W. R. BROWNE AND A. B. WALKOM. 



Plate XXVII. , Fig. 3. Rhacopteris tuff, showing miniature 



faulting ( x £). 



Plate XXVIII. „ 4. View of Drake's Hill and Post Office Hill 



from New Matthews' Gap Road. 



„ ,, 5. View looking towards Mount Bright from 



"Maluna." 



Plates XXVI and XXVII from polished specimens kindly lent 

 by Mr. W. Eustace Wilkinson. 



The VALUE of the NITRATE FIGURE in DETER- 

 MINING the FITNESS OP WATER for DRINKING 

 PURPOSES. 



By C. S. Willis, m.r.c.s., d.p.h., etc. 



Principal Assistant Medical Officer of the Government of 

 New South Wales. 



With Plate XXIX. 



[Read before the Royal Society of N. S. Wales, December 6, 1911.'] 



The necessity for considering all the figures obtained by 

 the usual chemical analysis of a water before giving an 

 opinion as to its fitness for drinking purposes cannot be too 

 strongly insisted upon. One frequently finds, even at the 

 present day, text books and analysts taking one figure, e.g, 

 the albuminoid ammonia figure, as a standard, and stating 

 that the water is fit or unfit for drinking purposes according 

 as it contains less or more than so many parts of this par- 

 ticular substance per 100,000. Such a course necessarily 

 frequently leads to serious errors. There are, however, 

 exceptional cases where one has to give an opinion on one 



