565 
of Edinburgh, Session 1871 - 72 . 
streams rid themselves of the unnatural impurities introduced into 
them by sewage, and by some of the manufactures. But I am not 
aware that the process of clearing has been watched with care in 
circumstances altogether natural. It occurred to me, at anyrate, 
that we have in the Yarrow a most favourable opportunity for 
tracing this process in the case of a natural water of a remarkable 
kind, under the operation of natural causes alone. On the 8th of 
September, therefore, I examined the course of the Yarrow with 
some attention. 
In its descent from St Mary’s Loch, it is first joined by two 
unimportant rills, at that time nearly dried up by ten days of pre- 
vious drought. A mile and a half below its outlet, it receives 
from the north its largest tributary, the Douglas Burn, which 
drains a very hilly country about five miles and a half long and 
four miles wide. This stream, indeed, was at the time a small rill, 
compared with the strong body of water in the Yarrow. But it 
was interesting in this respect, that its water, containing more 
saline matter than the main stream, and possessing the hardness 
of 4*90 degrees, presented no colour at all, even when examined in 
a 16-inch tube. This last fact is remarkable, because the Douglas 
Bum comes very much from peat-topped hills, so that either the 
peaty water of floods soon runs out in dry weather, and spring- 
water is alone left, or the water clears itself by eremacausis, or in 
its upper course in the way in which purification seems to be 
brought about in the Yarrow. 
For, when I came to examine the Yarrow immediately above 
the junction of the Douglas Burn, I found to my surprise that the 
colour, which at the outlet was such as to render a porcelain basin 
invisible when sunk 8 feet only, was already so much reduced, in 
the course of a mile and a half, as to approach the faint hue of the 
waters of Loch Katrine and Loch Lomond. There was also a 
slight increase of salts, as shown by the ordinary liquid tests, and 
also by the hardness of the water having increased from 1*4 to 2*40 
degrees. 
A mile lower down another principal tributary, but inferior to 
the Douglas Burn, falls into the Yarrow on the right, the Altrieve 
Burn, which, however, I had not time enough to examine. Two 
miles further on a similar streamlet joins from the right, the 
