Joty—On the Volume Changes of Rocks and Minerals attending Fusion. 297 
considerable gravitational force owing to the very high density of lead. T'ait,* in 
speaking of capillarity, points out that ‘the almost perfect sphericity of the little 
bead formed on the end of a glass fibre which is held in a flame for a short time” 
enabled many of the triumphs of the modern microscope to be anticipated. 
The small size of the beads used in these experiments is of course a source of 
protection against gravitational distortion. The force acting per unit of length 
radially inwards on any great circle is inversely as the radius of the sphere. 
Small spheres must in some cases experience considerable compressive stress. 
Relief of this stress at higher temperatures will not therefore necessarily be 
attended with distortion due to gravitational effects. 
Any considerable downward shifting of the image will be attended with error. 
But it will be apparent from figure 7 that this shift must be so considerable as to 
be at once visible before appreciable error will be introduced. The circle struck 
on the camera screen will enable a shift of less than a millimetre to be perceived. 
But such a shift need not demand rejection of the experiment or even allowance 
for it to be made. 
Against the possibility of some error from the sources quoted are to be set the 
peculiar advantages which this mode of experimenting presents when dealing with 
such substances as the silicates. In large masses it is quite impossible to free the 
more acid magmas from bubbles. But it was found in most cases not difficult to 
accomplish this in the case of the extremely small quantities going to make a 
bead. Again, measurements which involve the use of a containing vessel introduce 
the difficulty of allowing for the expansion of this vessel at high temperatures. 
This difficulty is even less considerable than that of observing accurately the 
change of volume of the magma within the vessel. The observations in the present 
method are effected under the same conditions of ease as we measure the size of a 
body under the microscope. Of course in certain cases the ability to deal with so 
small a quantity of the substance will enable observations to be carried out which 
otherwise were impracticable. 
BASALr. 
The following are the observations which determine the curve of expansion of 
the basalt of the Giant’s Causeway, Co. Antrim, as given in Plate XVIII. 
The bead was melted in the oven directly, being introduced as a rough fragment 
attached to the suspending wire. Diameter on screen 3°54 inches. As 4°60 inches 
on the screen represent 0-07 inch closely, at the position of the bead, the actual 
diameter of the bead is 0°054 inch. Also 268 divisions of the micrometer 
* << Properties of Matter,” p. 231. 
