j W. Gibbs on the theory of Atomicities, 409 
when it is understood that the two planes 2-7 in albite are cubic 
fac 
es. 
7 In addition to the above dodecahedric characteristics of the 
| crystals of the Feldspar group, it may be added that the twins 
are twinned parallel either to dodecahedric or cubic faces. The 
most common in orthoclase are those parallel to 7-2, dodecahe- 
dric ; others are twinned parallel to O, also dodecahedric ; oth- 
ers parallel to 2-2, or a cubic face. And there are no other di- 
rections of twinning among the triclinic feldspars. 
In another number of this Journal I propose to consider fur- 
ther the isometric relations of the various systems of crystalli- 
zation, a subject partly elucidated in the last edition of my 
Mineralogy (1854, vol. i, pp. 196-204), and also in this Jour- 
nal for the same year, vols. xvii, and xviii. 
Art. XLV.—On certain points in the theory of Atomicities; by 
Wotcort Gress, M.D., Rumford Professor in Harvard Uni- 
versity. aoe 
In the application of the theory of atomicities to the expla- 
nation of the formation and structure of compound bodies two 
distinct views are held by chemists. While Kekulé and his 
followers maintain that the primary or essential atomicity of 
each element is invariable, Wurtz, Frankland and others ad- 
mit that an element may have more than one degree of satura- 
ting power, Wurtz in particular assigning no specific maximum 
or limit in any case. ‘The chemists of both these schools distin- 
guish between atomic and molecular combinations, differing only 
as to what combinations are to be regarded as molecular. Thus 
according to Kekulé pentachlorid of phosphorus 1s a molecular 
compound, P™Cl,+Cl,, while upon the other view It 1s atomic 
and has the formula P’Cl,. All chemists appear to admit that 
water of crystallization is in molecular and not in atomic com- 
‘Dination. 
To this view it may be reasonably objected that it creates two 
distinct species of chemical affinity, for if phosphorus in the 
of sodium be a saturated atomic combination, by what force 
does it unite with two atoms of water to form NaCl+2H,@ 
The line which this theory draws between molecular and atomic 
combinations is, moreover, a purely arbitrary one, as is clearly 
Seen in the case of the chlorids of phosphorus cited above. In 
Am. Jour. Sci.—Szconp SERiEs,VoL. XLIV, No. 132.—Nov., 1867. 
46 
