Notices of Memoirs—Dr. H. C. Sorby’s Address. 471 
miscuously and without some definite relation to the special conditions of the case. 
Very much depends upon their chemical composition. Some substances yield almost 
exclusively those of one group, and other substances those of another, whilst in some 
cases a difference in the rate of cooling and other circumstances give rise to variations 
within certain limits; and, if it were possible to still further vary some of the con- 
ditions, these limits would probably be increased. Thus, for example, the earliest 
deposition of crystalline matter from the glassy solvent is sometimes in the form of 
simple solid prisms or needles, but later on in the process it is in the form of com- 
pound feathery tufts; and if it were possible to cool the beads much more slowly 
whilst they are very hot, I am inclined to believe that some substances might be 
found that in the early stage of the process would yield larger and more solid crystals 
than those commonly met with. This supposition, at all events, agrees with what 
takes places when such salts as potassium chloride are crystallised trom solution in 
water. Some of my blowpipe beads prove most conclusively that several perfectly 
distinct crystalline substances may be contemporaneously deposited from a highly 
heated vitreous solvent, which is an important fact in connection with the structure 
of igneous rocks, since some authors have asserted that more than one mineral species 
cannot be formed by the slow cooling of a truly melted rock. The great advantage 
of studying artificial blowpipe beads is that we can so easily obtain a variety of 
results under conditions which are perfectly well known, and more or less completely 
under control. 
Artificial Slags.—I now proceed to consider the structure of slags, and feel 
tempted to enter into the consideration of the various minerals found in them which 
are more or less perfectly identical with those characteristic of erupted rocks ; but 
some of the most interesting, like the felspars, occur in a well-marked form only in 
special cases where iron ores are smelted with fluxes, seldom, if ever, employed in our 
own country, so that my acquaintance with them is extremely small. My attention 
has been mainly directed to the more common products of our blast-furnaces. On 
examining these, after having become perfectly tamiliar with the structure of blow- 
pipe beads, I could see at once that they are very analogous, if not identical in their 
structure. In both we have a glassy solvent, from which crystals have been deposited ; 
only in one case this solvent was red hot, melted borax, and in the other glassy, melted 
stone. Thus, for example, some compounds, like what I believe is Humboldtilite, 
erystallise out in well-marked solid crystals, like those seen occasionally in blowpipe 
beads, whereas others crystallise out in complex feathery skeletons, just like those so 
common in and characteristic of the beads. In both we also often see small detached 
needles, scattered about in the glassy base. These skeleton crystals and minute 
needles haye been described by various writers, under the names, crystallites, belonites, 
and ¢trichites. Though we have not the great variety of different forms met with in 
the beads, and cannot so readily vary the conditions under which they are produced, 
yet we can, at all events, see clearly that their structural character depends both on 
their chemical constitution and on the physical conditions under which they have 
crystallised. None of my microscopical preparations of English slags appear to 
contain any species of felspar, but several contain what I believe is some variety of 
augite, both in the form of more or less solid prisms, and of feathery skeletons of 
great beauty and of much interest in connection with the next class of products to 
which I shall call your attention, viz., rocks artificially melted and slowly cooled. 
Rocks Artificially Melted.—I have had the opportunity of preparing excellent 
thin microscopical sections of some of the results of the classic experiments of Sir 
James Hall. I have also carefully studied the product obtained by fusing and slowly 
cooling much larger masses of the basalt of Rowley, and have compared its structure 
with that of the original rock. Both are entirely crystalline, and, as far as I can 
ascertain, both are mainly composed of the same minerals. Those to which I would 
especially call attention are a triclinic felspar and an augite. The general character 
of the crystals is, however, strikingly different. In the artificial product a con- 
siderable part of the augite occurs as flat, feathery plates, like those in furnace slags, 
which are quite absent from the natural rock, and only part occurs as simple solid 
crystals, analogous to those in the rock, but much smaller and less developed. The 
felspar is chiefly in the form of elongated, flat, twinned prisms, which, like the 
prisms in some blowpipe beads, commence in a more simple form, and end in complex 
fan-shaped brushes, whereas in the natural rock they are larger than in the artificial, 
and exclusively of simple character. On the whole, then, though the artificially 
