288 Joty— On the Volume Change of Rocks and Minerals attending Fusion. 
bead of platinum prepared in the oxyhydrogen flame by fusing the extremity of a 
fine wire till a bead of sufficient size had been formed. Care was taken to give this 
bead the dimensions that it was intended to confer upon the fused mineral beads 
to be in the future dealt with. This platinum bead being suspended freely in 
the centre of the oven, and the image sharply focussed on the screen, observations 
of its diametral dimensions were made under increasing strengths of current. 
Here the temperature is calculable from the amount of linear expansion observed. 
The readings of the galvanometer plotted against these temperatures afford a curve 
to be availed of in the after-use of the oven. Very consistent results were 
obtained by this method. Small variation in the dimensions of the sphere of 
platinum or in the thickness of the wire suspending the sphere appeared to make 
no appreciable difference injthe curve. From one oven to another the results 
were nearly the same, although at this time the ovens were only roughly moulded 
on a wire, and the special precautions subsequently taken to secure that they 
were alike in all dimensions were not then taken. This method of temperature 
calibration was subsequently abandoned owing to a reason to be presently given ; 
the results of the observations are, however, worthy of record. 
The dimensions in what follows are given in inches. The diameter of the 
platinum bead or thermometer was in one set of experiments 0:06687. This was 
ascertained as follows :—A glass slip divided to hundredths of an inch was placed in 
the field of the object-glass. It was found that eight divisions on the glass were 
magnified to 5:22 inches on the screen, showing that the magnification was 65:2 
diameters. The projection of the globule of platinum measured 4°36 inches. _ Its 
real dimension is ascertained from these data. Referring measurements to the 
bead and not to its image, it is further found that the reading microscopes require 
the cross wire to be moved through 276 divisions of the micrometer bead for 
0-001 inch on the divided slip. If now JV is the number of divisions of the head 
* . . . . . 
corresponding to the increase of diameter of the bead at any observation, and if 
f, and L, are the first and last diameters of the bead, then Z, — L, = —— 
and this number divided by Z, and by the coefficient of expansion of platinum over 
this range of temperature will afford the temperature of the oven. The tempera- 
tures corresponding to the expansions were directly obtained from the observations 
with the meldometer which I have recorded in the Proceedings of the Royal Irish 
Academy.* These observations afford the extension of a platinum ribbon when 
bodies of well ascertained melting points are melting in fine adherent particles 
upon its surface. In this manner the following points upon the required curve of 
galvanometer readings and temperatures within the oven were ascertained. 
* Proc. R. I. A., 3rd ser., vol. u., p. 38. 
