_ Tungstate... PbTu 
ON DIMORPHOUS BODIES. 197 
sulphuret for example, may be made to assume by bringing 
more powerful chemical affinities into operation? Bicyanide 
of mercury is completely decomposed by dry sulphuretted hy- 
drogen; bichloride only on the surface. If the latter be pre- 
viously moistened, it is entirely decomposed ; but during the 
action of the gas, it gradually falls to powder. The phenomena 
in the latter case are owing to the existence and previous form- 
ation of a compound of the two salts, a sulpho-chloride, and 
not, necessarily, to any inability of the bisulphuret to assume 
and retain the form of the bichloride; yet it is not impossible 
that there may exist some unknown relation between the true 
form of a body and those false forms which it is capable of as- 
suming and retaining in any degree of perfection. 
VII. 
43. Of Trimorphous hodies—Though we are as yet unac- 
quainted with any cases in which bodies actually assume more 
than two incompatible forms, yet, as I have already remarked, 
thereisno reason to consider such an occurrence as at all unlikely. 
On the contrary, there are strong reasons for believing that future 
observations will make us acquainted with three or more forms 
of the same substance, geometrically distinct. ‘The analogous 
compounds, for example, of isomorphous bodies ought to as- 
sume the same form, and yet we are familiar with many groups 
of such compounds which, though their individual members 
are not known to assume more than one or two irreconcileable 
forms, yet, as groups, are tri, or even tetrakimorphous. In 
a former section I have illustrated, by reference to one or two 
cases, in what way the probable dimorphism of individual com- 
pounds may be inferred from that of the chemical group to 
which they belong; the same mode of deduction renders tri- 
morphism almost equally probable. Thus the sulphate, chro- 
mate and molybdate of lead, present us with three forms: 
Sulphate... PbS aRt. Rh. Pr. 103° 42! 
Chromate...PbC Ob. Bh. Pr. 93°30! 99° 10! 
Molybdate . . bee Bias aries 
exhibited by substances represented by the same general for- 
mula RR, and which, for anything we know to the contrary, 
may all be assumed by each other. 
