Joty—On the Volume Change of Rocks and Minerals attending Fusion. 287 
The ordinary camera screen of ground glass must be replaced by one of far 
finer grain. That finally adopted was prepared as follows:—A sheet of clear 
glass was heated and flooded with melted paraffin wax. At the two areas where 
observation is to be directed large cover glasses of the thinness of that used for 
covering microscopic objects, and some 4 or 5 centimetres in diameter, are pressed 
down over the melted paraffin, expelling most of it from beneath the glasses. 
When cold there are thus two areas upon the screen where the texture of the 
paraffin is specially fine. The fineness is increased by cooling the paraffin rapidly. 
This is done by touching the clean side of the plate down upon the surface of cold 
water, while the paraffin is still melted. When cold a circle is struck with a compass 
upon the paraffin. The circle is so placed that it traverses the cover-glasses 
centrally, but, of course, is not impressed upon them, but only on the paraffin 
around them. This circle is 11 centimetres in diameter, and marks the dimensions 
and position to which the image of spherical beads of the substance is to be 
brought. The object of this fixed circle on the camera screen is an important 
one. When observation is in progress a source of error arises if the image shifts 
appreciably in the field of the micrometer microscopes. ‘The fixed circle enables 
such a shifting to be detected at a glance. The 
accompanying figure (fig. +) further explains the 
nature of the special screen just described. 
In carrying out the experiments the utmost 
steadiness attainable must be secured in the fixing 
of the train of apparatus. A stone pier with slate 
top was, in the case of the experiments herein 
described, used to carry the entire train. To the 
edge of the slate top the reading microscopes were clamped. Care must be 
taken to adjust these at such equal distances from the screen that the readings of 
both divided circles possess equal values, or if different the difference must be 
ascertained and allowed for. 
The heating of the oven, and the mode of determining the temperature within 
it were effected as follows:—The current was supplied from a storage battery, 
and was controlled by a resistance handily adjustable while making observations. 
Lord Kelvin’s graded galyanometer was used to observe the current. <A current 
of 15 ampéres, and even more than this, was occasionally employed. In practice it 
is not, however, necessary to determine the amount of current traversing the oven. 
It was found most convenient to set the sliding table at IV. on the scale of grades, 
and simply record the deflections of the needle when making the observations for 
the determination of temperature in the manner to be described. 
The first mode employed to determine the temperature within the oven was to 
hang within it, supported from the cross wire in the ordinary manner, a spherical 
Fic. 4. 
