248 Prof. Huxley on the Structure of Glacier Ice. 
hence the thickness of the superficial layer) varies a good deal ; 
7 or 8 inches is perhaps rather above than below their average 
depth ; but however this may be, the important fact is, that 
whenever you clear away the superficial layer, you find beneath 
it what I have termed “ deep ” ice—that is, ice in which neither 
fissures nor granules are discernible; ice which tends to split 
parallel to the veins, and shows no disposition to break up into 
the angular fragments so characteristic of the superficial layer. 
It has been said that mere optical examination is insufficient 
to disprove the existence of fissures in the deep ice, and that 
such fissures are present, though invisible in consequence of 
being filled with water. I have already shown that the line of 
contact of water and ice is optically well marked, and that there 
is every reason to believe that even the finest fissures would be 
visible under a sufficient magnifying power; but those who 
maintain the porosity of glacier ice, rest chiefly on the results 
of experiments made with coloured fluids. It is said that glacier 
ice becomes infiltrated throughout its substance with extreme 
readiness, the coloured liquid traversing fissures which are more 
particularly developed in the course of the blue veins. It be- 
came necessary, therefore, to repeat these infiltration experi- 
ments ; and for this purpose, as you will recollect, I made use of 
the logwood infusion which you had prepared, and which by its 
combined clearness and intensity of colour was excellently fitted 
for the object in view. 
If a little of the infusion were poured upon the natural sur- 
face of the glacier, it immediately soaked in, spreading itself in 
all directions between the granules (but more rapidly, as I often 
observed, in directions parallel with the veins), and staining the 
whole thickness of the superficial layer. Whatever quantity 
might be poured on to the surface, however, it penetrated no 
further than the superficial layer (unless there were some ob- 
vious crack in the deeper ice); and when the latter was cleared 
away with the axe, and the surface of the deep ice washed or 
even carefully rubbed with the hand, not a trace of the infusion 
could be found in it. 
If a piece of the deep ice containing several blue veins were 
allowed to soak in the logwood infusion until it nearly melted 
away, it remained unstained, and either wiping it or passing it 
quickly through clean water rendered it perfectly clear and 
stainless. 
But it is said that if cavities be made in the glacier and filled 
with a coloured infusion, the latter will soon, by means of the 
capillary fissures, infiltrate the surrounding mass. To deter- 
mine this point, I selected a spot upon the north wall of a cre- 
vasse, just opposite the Montanvert, and between the centre 
