56 
DR. A. E. H. TUTTON ON THE 
two non-eutropic cases with surprisingly slight alteration of the morphology of the 
crystals, the change, in fact, being almost nil when rubidium is the alkali metal 
replaced by either ammonium or thallium. While no clear rule like that of eutropic 
progression can apply as regards ammonium (or thallium), there are still very definite 
relationships which have been established as the result of the research now completed, 
which can best be stated separately for the various properties. 
Crystal Angles .—-The monoclinic axial angle (3 of the ammonium salt of any group 
(set of salts containing the same M-metal) is usually very close to that of the caesium 
salt of the same group. In six of the eight groups, those in which all four salts (of 
NH. ( , K, fib, and Cs) are capable of existence, the difference is less than 10', and in three 
cases it does not exceed 2'. The change in this important angle when K is replaced 
by Rb is usually about a degree, and when Cs replaces K about 2 degrees, the latter 
being almost exactly twice the former, the angular changes corresponding to the two 
changes in atomic weight, 46 and 93. The maximum change's observed for the two 
replacements were T 12' and 2° 20', in the iron group, the NH 4 replacement in this 
group causing 2° 18' of change. 
In all the six complete groups 32 to 34 out of 36 different interfacial angles 
compared (all in which the differences are beyond the range of experimental error) 
show a change of angle on replacing K by NH 4 in the same direction as if Rb or 
Cs were introduced instead. The average and maximum amounts of the change when 
NH 4 replaces K are approximately the same as when K is replaced by Cs, and this 
amount is twice as great as when Rb replaces K ; for the average and maximum 
changes of angles are directly proportional to the change of atomic weight when one 
alkali metal is substituted for another. (K = 38'85’ when H = 1, Rb = 84’9, and 
Cs = 131’9, and ,J S 85 + TR 9 _ . a j g0 pq, —K — an d Cs —Rb = 47.) The 
maximum change of angle observed for the replacement of K by Cs occurs in the 
magnesium group, and is 2° 25'; the replacement of K by NTI 4 in the same group 
provokes a change of nearly the same amount, 2° 18', while the change for the 
replacement of K by Rb is exactly half the value for the Cs replacement, namely, 
1° 12'. This direct proportionality of the average and maximum change of angle to 
the change in atomic weight of the alkali metal is one of the most striking and 
important of all the’ facts brought to light by this prolonged investigation, and is 
rendered the more important by the large amounts of these changes. It is also 
interesting to note 'that the change (strictly proportional to that in atomic weight) 
in the most important of the angles, the axial angle (3, between the primary 
orthopinakoid a {100} and basal plane c {001}, is so large as to be nearly equal to the 
maximum, and in several groups this angle is actually itself the angle of maximum 
change. These conclusions are, therefore, far away from any possible experimental 
error, which at the highest estimate could not exceed 5' and probably does not 
exceed 3'. 
