284 Joryv—On the Volume Change of Rocks and Minerals attending Fusion. 
enlargement of the object, be kept upon the movable spider-lines of the 
micrometers, and the amount of enlargement measured over extreme changes 
of temperature. Or a photograph may be taken both when the substance is cold 
and when it is heated, and the amount of expansion of the image may be 
measured upon the photographs. 
To effect the heating of the substance while thus maintained in the field of 
the microscope might appear to present great difficulties. The difficulties were 
surmounted by the use of an oven consisting of a ribbon of platinum which could 
be heated to any required temperature by passing a current through it, and which 
was folded in such a manner round the substance that this occupied the centre of 
a heated tube, of considerable length relatively to the dimensions of the substance, 
and so placed that the light employed to illuminate the object entered at one end 
of the tube while the rays going to the lens emerged at the other. The observer 
thus looks axially into the tube. The axis of the tube was maintained horizontal, 
the microscope serving to project the image into the camera also being in a 
horizontal position. 
Before entering into details a few numbers showing the attainable degree of 
accuracy will be of interest. 
The case of observations on the expansion of diamond, in which the magnifi- 
cation was by no means so high as was in some cases found practicable, will show 
the magnitudes dealt with. The diamond in question was rather less than 2 
millimetres in diameter, that is as measured from one extreme solid angle of the 
octahedron to the other. It was so supported in the field of the microscope that 
two opposite extremities of the octahedron were as sharply as possible focussed 
upon the camera screen. From one extremity of the image to the other 
measured closely 11 centimetres. A movement of the wires of the reading 
microscopes by one division of the head of the micrometer in following the 
expansion of the image corresponded to a diametral enlargement of this latter 
by 0:0005 cm., but owing to imperfect focus of the image no more than 0-001 cm. 
could be accurately determined. This it will be seen is the 1/11000 part of the 
linear dimensions of the image. If the coefficient of expansion of the substance 
were that of glass or that of platinum (about 0-00001), the expansion of the image 
due to a change of temperature of 10° C. would be fully determinable. This 
change of temperature would in fact produce a linear expansion of the image 
by the 1/10000 part of its diametral length. 
It will be apparent that as the measurement we are concerned with is a ratio, 
that of the linear dimensions of the body when hot to its dimensions when cold, 
it is only necessary in the course of experiments to observe the thermal expansion 
of the image, if the expression is allowable. The ratio of the first and last 
dimension of the image being the same, of course, as the ratio of the first and last 
