549 
of Edinburgh, Session 1871 - 72 . 
2000 feet high, “ The Cruach ” presents an imposing, rugged, 
conical sky-line to one entering Loch G-oil from Loch Long. The 
east face, precipitous at the summit, is entirely grassy lower down, 
unless where broken by other precipices, out-cropping rocks, or 
stream-courses, also always rocky. There is little peat to be seen 
anywhere, and no agriculture. From various trials around Loch 
G-oil and Loch Lomond I am satisfied that this streamlet is a fair 
type, both in its ordinary state and in its occasional variations, of 
most of the streams which tumble into these sheets of water from 
the mica-slate mountains around them. 
When I examined this water in the end of September, after ten 
days of perfectly dry weather, following a heavy twelve-hours’ rain 
two days earlier, it was beautifully clear and sparkling. In the 
first place, it was entirely free from colour. The absence of colour 
was tested conveniently and delicately by means of a glass tube 16 
inches long and six-tenths of an inch in diameter, which is nearly 
filled with the water to be examined, and is held over, but not 
touching, a sheet of white paper in a bright light. For security, a 
very fine colourless spring water was always kept at hand for com¬ 
parison in another tube. The slightest coloration is thus seen by 
looking perpendicularly down the tube. Or it may be equally recog¬ 
nised by looking at the surface of the water obliquely through the 
upper part of the tube from a distance of 18 inches or 2 feet; for the 
colour is thrown up by the paper, and concentrated, as it were, on 
the surface of the water, though the long subjacent column, as seen 
through the glass, appears colourless. Very few waters, except 
that of springs, withstand altogether this test of the presence of 
colour.* Mr Dewar has suggested that it admits of being made a 
water-chromometer, by employing for comparison,—distilled water 
being used for fixing the zero point,—a solution of some invariable 
strength of a permanent per-oxide salt of iron, such as the acetate, 
and diluting the solution to uniformity of depth of colour with the 
water to be compared. The amount of dilution would denote the 
degree of coloration relatively to a fixed standard. 
In the second place, this water contained a very small propor- 
* This method, devised for the occasion, I have since found to be a mere 
variety, but more[convenient, of one proposed some years ago by Ur Letheby, 
and adopted by the late Professor Miller. 
