NORMAL SULPHATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 
465 
to trace an image; it is of such focal length that when it is close up to the eyepiece 
it permits the image of the signal to he seen almost as well as when it is absent, 
while as it is drawn more and more in front of and away from the eyej)iece it causes 
the image to pass gradually into that of the jiarticular reflecting face of the crystal 
itself The latter is clearly focussed when the sliding tube has been drawn forward 
to the full extent of its path, and the face afibrding the signal image is seen brightly 
illuminated, as well possibly as other vicinal faces, from which it is distinguished by 
the tracing process just indicated. 
The rest of the arrangements of the instrument are precisely as described in the 
former memoir {loc. cit.). 
Procedure in Cutting and Grinding the Crystal-hlochs. 
In selecting crystals from which to prepare a parallel-faced block, those were 
naturally chosen which were free from traces of turbidity and from cracks and 
distortions. Crystals of caesium suljehate are readily obtained perfectly free from 
turbidity; in the case of rubidium sulphate only very slow growth in vacuo yields 
crystals satisfactory in this respect. The exceptional crystals of the potassium salt, 
eventually obtained after so much trouble, as has been referred to, were also satisfactory 
from this point of view. 
After removal from the mother liquor, the crystals were carefuUy dried, and then 
immediately stored in a desiccator for several days at least before iise. With two 
exceptions each selected crystal was only employed for duplicate determinations, on 
two successive days, of the linear thermal expansion or contraction along some one 
particular axial direction. In aU, 29 difierent crystals were employed, 11 of 
potassium sulphate, 8 of rubidium sulphate, and 10 of caesium sulphate. The two 
exceptions were crystals of the rubidium and caesium salts, the former of which was 
a particularly fine specimen elongated along one axial direction, and which, when 
cut in two halves transversely to this direction yielded portions so large that they 
were separately employed for determinations in two different axial directions; the 
crystal of caesium sulphate was cut and ground into a rectangular block for successive 
determinations in all three axial directions, so as to afford an instance of all three 
linear values, and from these the value for the cubic expansion, being derived from 
one and the same crystal, for comparison of the cubic deformation thus obtained with 
that derived by calculation from measurements of the three linear expansions or con¬ 
tractions exhibited by difierent crystals. The results were so nearly identical, and 
the comparison therefore so satisfactory, that there will be no occasion to further refer 
to this point. 
The orientation of the various faces, and the consequential identification of the 
axial directions of the crystals, was usually an easy matter, as the author was familiar 
with the salts in question owing to the exhaustive morphological and optical study 
VOL. cxcn.—A, 3 o 
