232 J. P. Cooke on the Law of Definite Proportions 
A theory that the variation in composition resulted from the 
mixture of two or more isomorphous compounds would be even 
Jess tenable than the one just discussed. For in the first place it 
would be necessary to assume the existence of two other com- 
pounds of zinc and antimony isomorphous with Sb Zn2 and of 
one other, if not more, isomorphous with Sb Zns. Not only 
would such an assumption be contrary to all the analogies of chem- 
istry and therefore require strong evidence to sustain it; but in 
the second place it can almost be demonstrated that no such com- 
pounds exist. ‘The crystals having the calculated composition of 
either Sb Zns or Sb Znz are marked as has been shown by stri- 
king peculiarities, and with one possible exception similar pecu- 
liarities were not observed throughout the whole series of erystals 
which have been examined. The crystals containing 50 per ct. of 
zinc and of the composition of Sb Zns were found to havea 
slightly smaller sp. gr., than those just above or just below them, 
but the difference is so small that it may be accidental, and as the 
crystals exhibited none of the other peculiarities, which charac- 
terize crystals having the calculated composition of Sb Zns or 
Sb Zna, I could not attach sufficient weight to the one cireum- — 
stance to feel authorized in admitting a third compound of zine 
and antimony. Admitting however the existence of Sb Zns 
yet, as exactly the same angle has been observed in crystals con- 
taining 55 per cent. as on those containing 43 per cent of zine, 
it would be necessary in order to explain the variation in compo- 
sition by the principle of isomorphous mixtures, to assume the 
existence of still a third compound isomorphous with Sb Zns, 
and containing more zine than Sb Zna, which would increase 
can not be explained either by mechanicafsimpurities in the crys- 
tals or by the mixture of isomorphous compounds. 
