S Dr Brewster on the Structure Apophyllite. 
The appearance exhibited by the cruciform harmotome is shewn 
in Fig. 11, where each of the compressed dodecahedrons is re- 
duced as it were to nothing at its axis, by two opposite grooves, 
in order to admit of a symmetrial combination. Now, it is quite 
obvious, that the only difference between this compound crystal 
of harmotome, and the compound double pyramid of sulphate of 
potash, is, that the former is composed of two crystals, crossing 
one another at angles of 90®, whereas the latter consists of three 
crystals, crossing one another at angles of 60 The combined 
crystals of Harmotome, however, have not the remarkable pro- 
perty of forming one of the simple primitive forms of crystals, 
as is done by the combined rhomboids of sulphate of potash. 
In various other crystals, I have observed phenomena that 
have some analogy with those which are the subject of this pa- 
per. In Nitre, they are very common, so as to exhibit two ad- 
jacent triangles turned round 30® ; and I have noticed a very 
reinarkable appearance of the same kind in a specimen of sul- 
phate of lime from Montmartre, which contained a film of the 
form of an oblique parallelogram turned round 25®, while all 
the films around it had suffered no change in the position of 
their axes. 
The results which I have now endeavoured to explain, possess 
a considerable degree of interest in their optical relations. Their 
influence upon the doctrines of crystallization is very obvious, 
and the mineralogist will not scruple to admit, that, if the physi- 
ology of mineral bodies shall ever attain the dignity of a science, 
its foundations must be laid upon optical results, and its progress 
directed by the unerring light of optical analysis, 
Edinburgh, Jan. 29. 1819. 
Art. II. — -On the Hyposulphurous Acid and its Compounds. 
By J. F. W. Herschel, Esq. F. B. S. &c. Communicated 
by the Author. 
The experiments about to be described, were occasioned by 
the following accident. Having set aside, for a few days, a so^ 
lution of hydroguretted sulphuret of lime, I was struck by ob- 
serving a bitterness in the liquid^ when almost wholly decom^ 
