466 ME. A. E. TUTTON ON THE THERMAL DEFORMATION OF THE CRYSTALLISED 
already made. The identification of the axial directions is such a vital matter, as 
will be abundantly evident when the results are discussed, that it should be stated 
that the whole of the work, in common with all the author’s previous crystallo- 
graphical investigations, has been carried out exclusively by the author personally. 
In every case, the axial directions were actually verified both by goniometrical 
measurements and by examination of the interference figures in convergent polarised 
light. Immersion in a cell of benzene, on the inverted goniometrical polariscope, 
materially facilitates the latter verification, as the refi’active index of that liquid is 
not far removed from the mean of the indices of the three salts, and the interference 
figures are consequently very clear, and the apparent optic axial angle is very nearly 
the true angle. Oil cannot be used, as it is apt to penetrate into any minute cavities 
in the surfaces, and to ooze out during the thermal observations in drops too small to 
be noticeable without a lens, but which are sufiicient to entirely derange the inter¬ 
ference bands by lifting the compensator by an amount which is very appreciable in 
observations of such delicacy. 
For the same reason oil cannot be used in grinding the surfaces, and recourse was 
again made to benzene, which by its volatility rapidly removes itself from cavities. 
It is naturally unavoidable that greater quantities require to be used than of oil, as it 
so rapidly dries away. Hence a dropping funnel was arranged above the cutting or 
grinding disc, to deliver drops sufliciently fast to continually provide adequate lubrication. 
The crystal, after verification of the axial direction along which it was desired to 
determine the linear deformation, was mounted in the grip-holder, in the manner 
already described, with the axial direction in question apjDroximately vertical, parallel 
to the goniometrical axis and perpendicular to the cutting disc and grinding table. 
The apj^roximation was then converted into absolute adjustment, by goniometrical 
observation and adjustment of the natural zone of faces parallel to the axis in 
question. If the crystal were so terminated below that much grinding Avould be 
necessary to produce the required surface, the cutter was first brought into requisition 
and the lower end cut oft*, at such a distance as to aftbrd a surface of the requmed 
extent with the least sacrifice of thickness in the axial direction adjusted. The 
cutting of these crystals of artificial salts, although they are so much more friable 
than mineral crystals, is nevertheless most successfully performed by the new instru¬ 
ment. No crystal has yet been broken in the process. If no cutting were required 
the rough grinding of the surface Avas carried out on the emery-cloth lap, at first with 
the crystal-holder and lap detached from the insti'ument and tlie former held in the 
hand, and tlieii, after a rough approximation to the desired surface had been attained, 
with the crystal and its holder and the lap in position. Finally, after verification of 
the adjustment, which, owing to the mode of fitting of the holder on to the suspended 
adjusting apparatus, Avas usually unimpaired, the surface Avas finely ground AAuth one 
of the ground-glass laps. 
As the method of using the aluminivim compensator aboA'e the crystal AA'as alAA’ays 
