Joty—On the Volume Change of Rocks and Minerals attending Fusion. 293 
possible avoided as probably attended with the break-up and loss of some of the 
fundamental constituents of the magma. The intermediate magmas are not dealt 
with in this Paper. The nearest representative in composition dealt with is the 
fusion product of the mineral augite. This has probably only some 50 per cent. of 
silica, thus falling short of the silica percentage of a basic-intermediate magma 
by some 5 per cent. of silica. Its alumina percentage is deficient and its mag- 
nesia and lime percentage excessive. Finally, the basic magma was directly 
dealt with in the form of a fused fragment of the basalt of county Antrim. 
This basalt is so fine-grained that its crystalline nature is only apparent to the eye 
upon very careful examination, and a small fragment may be taken as fairly 
representative of the rock. 
From what has already been stated the mode of dealing with these bodies will 
be readily inferred. A bead is prepared by fusion of a small fragment of the 
substance. This is caused to adhere to the extremity of a very fine platinum wire. 
The wire being cut off and bent into a hook, in the manner depicted in figure 2, 
the bead is suspended from the cross wire of the heating apparatus, and adjusted 
till it is centrally placed within the oven. A sharp image of the spherical bead is 
now formed on the camera screen, the wires in the reading microscopes brought 
accurately into tangential contact with the image, and the reading of the micro- 
meter heads noted down. The gradual rise in temperature of the bead is now 
brought about by a sliding resistance, which controls the current in the oven. 
At desired stages in the heating the galvanometer is read, and the wires of the 
micrometer readjusted upon the edge of the image, the readings of both divided 
heads being noted. Finally a temperature is attained when the bead no longer 
retains its spherical shape. Observations, then, of course come to an end. Stop- 
ping short of this a few readings may be taken from the galvanometer and micro- 
meters while reducing the temperature of the bead. Before entering further into 
details as to the nature of these observations it is necessary to describe the mode 
of preparing the fused bead. 
Two different methods may be resorted to in fusing the minute fragment of 
the substance. The oxyhydrogen flame may be applied directly. This will bring 
the more difficult fusible bodies rapidly into a liquid state. A certain amount of 
sodium will be visibly driven off. Boiling will ensue and last for a few seconds 
or for several minutes according to the nature and the degree of viscosity which 
the substance still retains. Finally a smooth bead will remain. The manipula- 
tion involved in this operation is very simple. ‘The rough fragment of the 
substance is placed on a slip of platinum and the flame brought down to it, while 
at the same time it is approached with a fine platinum wire. Directing the point 
of the flame at the point of contact of the wire with the surface of the mineral the 
mineral is fused to the extreme end of the wire. Holding the wire vertically, the 
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