386 Observations on Polluted Waters . [July, 
anywhere, nor had they ever before, within the bounds of 
tradition, received visits from the N.W. 
Some years before my arrival a large body of pirates had 
made a sudden descent on their islands, and carried off 
nearly all the inhabitants. Those who escaped did so by 
paddling out to sea, and remaining there some days, keeping 
the tops of the trees just within sight. This expedition, as 
I learnt afterwards, had been sent by the Sultan of Zidore, 
and was not considered by the Dutch as an a6t of piracy ; 
and I believe some correspondence took place with the 
English Government in consequence of the late Sir Edward 
Belcher having been of a different opinion. 
St. David’s, or Free Will, Islands are— or rather the 
largest of them is — called Mapia. 
III. OBSERVATIONS ON POLLUTED WATERS. 
By J. W. Slater. 
t ^HE self-purifying power of polluted rivers, and still 
^ more of town sewage, has been the subject of no 
little discussion, and is still far from having been 
brought to a decision. I have, in a former volume of the 
“ Journal of Science” (1881, p. 138), referred to the oppo- 
site conclusion, which has been reached by the late Royal 
Rivers Pollution Commissioners, on the one hand, and, on 
the other, by Dr. Tidy, and by a French chemist whose 
name has escaped me, but who made a very minute study 
of the waters of the Vesle, above, at, and below the town of 
Rheims. 
The first-mentioned authorities, — perhaps we should rather 
say authority, — without declaring in general terms that a 
river once contaminated could not purify itself again, main- 
tained that at any rate no such restoration was possible 
within the length of any river in the United Kingdom. Dr. 
Tidy is of a different opinion, and the French chemist above 
referred to gives an instance where the gradual change from 
purity to pollution, and thence back again to purity, maybe 
seen by anyone who is willing to spend a day in examining 
