286 JoLty—On the Volume Change of Rocks and Minerals attending Fusion. 
wire is necessary, or there is risk that a vibratory motion of the bead on the least 
provocation would render observation impossible. An idea of the actual dimen- 
sions of the several parts of the apparatus is obtained from figure 1, this repre- 
senting the apparatus to actual size. 
In order to secure that any particular oven when worn out can be replaced by 
another oven of the same dimensions, the following mode of making the ovens 
was finally adopted. It will be seen later that this equality of dimensions must be 
secured if the thermal calibration of one oven is to be applicable to another oven 
replacing it. 
Figure 3 shows, much enlarged, a small length of brass tube attached to the 
face of a vertical plate, and having a fine saw-cut 
made longitudinally on one side. A short length of 
smooth brass wire having the diameter of the in- 
terior of the oven is shown within the tube, also 
supported by attachment to the plate. A fold of 
the platinum ribbon used to form the ovens is 
passed round it, the ends of the ribbon being pulled 
through the slit in the tube, and bent downwards 
round the outside of the tube, being passed under 
Fic. 3. 
the wings, and finally is cut off at the extremity of the latter. Carefully removing 
the ribbon from the tube, the completed oven is obtained. A number are made 
at the one time and preserved for use. Their position when in the forceps is 
shown in figure 1. The ribbon is 8 mm. wide. 
The mode of illumination in a great measure determines the degree of accuracy 
finally attainable. Many illuminants and optical trains were tried before that 
finally adopted was determined on. 
The source of radiation is a small pencil of hard lime. This pencil is about 
2 millimetres in diameter, and is raised at one extremity to whiteness in the 
oxyhydrogen flame. The lense used to condense this light upon the substance 
receives rays only from this minute radiant. The pencil rests upon a lime cylinder 
placed in its usual position in the ordinary limelight apparatus, the usual non-mixing 
jet adjusted to a fine point being brought to play on the extremity of the pencil. 
The optical arrangements embrace, first, a good condenser of the ordinary 
magic-lantern type. This is carried in a tube which is freely adjustable as 
regards its distance from the radiant. The rays then enter the oven and fall 
upon the substance. This is magnified by a one-inch object-glass. The ,best for 
the purpose was found to be that of Messrs. Newton, as made for their projecting 
microscope. The eyepiece of the microscope is removed, the required enlarge- 
ment of the image being obtained by lengthening the camera. Stray light is 
carefully screened from the interior of the camera. 
