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XII. 
ON THE VOLUME CHANGE OF ROCKS AND MINERALS ATTENDING FUSION. 
By J. JOLY, M.A., B.A.I., Se.D., F.R.S., Hon. See. Royal Dublin Society. 
(Prates XVII. anp XVIII.) 
[Read May 19, 1897.] 
Ir is now some years since I exhibited before the Royal Society of London the 
method and experiments described in the following pages :—The exhibition finds 
record only in the Catalogue of the Exhibits (June, 1892), the mode of experi- 
menting finds but a scant explanation, and the results are necessarily not 
recorded at ali. Later (Nature, March 22, 1894), a short account of the method 
in connexion with its application to determining the thermal expansion of 
diamond appeared. 
The experiments were thus held over in the hopes that leisure would be found 
to extend them. But as the value of the method as affording closely approximate 
results is, I venture to think, beyond doubt, and as it may still be long before the 
opportunity to extend the results arises, I have thought it best to give here an 
account of the work already accomplished. The question whether the volume 
change upon fusion of certain important rocks is positive or negative in character 
finds an undoubted answer in the results here recorded. This answer is not of 
the less value because it confirms the results of other observers. The method avoids 
sources of error to which older methods are open. But not only is this important 
question answered definitely, but also the course of the volume change from air 
temperatures up to those above softening is indicated. There is also the added 
reason for the publication of the method that others may possibly avail them- 
selves of it, seeing that it is applicable generally to the problem of determining 
the volume-change of minute quantities of a substance over high ranges of 
temperature. 
The method is very simple. The small fragment of the substance to be dealt 
with is placed in the field of a microscope, and an image of the fragment projected 
into a photographic camera, where it appears upon the focussing screen magnified 
about 80 or 90 diameters. This image is observed at opposite points by two 
micrometer microscopes, so that opposite edges of the image may, upon any 
TRANS. ROY. DUB. SOC., N.S. ‘VOL. VI., PART XII, 2X 
