98 Mr. Maskelyne on the Crystal Molecule [April 1, 



symmetry. Thus rhombohedral crystals exhibit a triple acoustic sym- 

 metry round the morphological axis ; and the axes of acoustic elasticity, 

 appear to coincide with the crystallographic axes, 2.e., with the intersec- 

 tions of the planes of the fundamental rhombohedron . In some cases the 

 the acoustic characters of the crystal (indicated by the tones produced, 

 as also by the lines into which finely sprinkled particles arrange them- 

 selves on a plate of the substance when set vibrating by a violin bow), 

 seem to provide a more subtle means of investigating its elastic struc- 

 ture than the coarser methods of cleavage and hardness [Savart*s 

 Experiments — this is especially exemplified in the case of quartz], 

 and crystal acoustics would, in this respect, seem to offer a fertile field 

 for investigation. 



In the Magnetic relations of the several parts of a crystal much 

 has been done [Faraday, Plucker, Tyndall and Knoblauch, Grailich 

 and Von Lang]. The results of Professors Tyndall and Knoblauch 

 have been completely confirmed and extended by Grailich and Von 

 Lang, who have added to their results a large series of magne-crystal- 

 line determinations, and expressed them by a nomenclature^ at once 

 comprehensive and concise. Tyndall and Knoblauch have shown that 

 the intensity of the dia-magnetic {i.e., equatorially tending), or of the 

 paramagnetic (i.e., axially tending) action, depends on the density of 

 the substance along particular directions. The Vienna observers 

 prove that isomorphous bodies, of similar (dia- or para-) magnetic kinds, 

 have a similar magnetic set ; and that those of opposite kinds (one of 

 para- the other of dia-), magnetic nature, comport themselves in ways 

 precisely inverse to one another, and that thus the magnetic orien- 

 tation* depends upon the different relative densities of the crystal 

 along different crystallographic directions in it. 



There is evidently here a qualitative action (an axial or an equa- 

 torial set) which is to be distinguished from the amount of energy, 

 of this setting or directive force, in various directions of the crystal. 

 The former depends on the chemical nature of the substance ; the 

 latter solely on the crystalline arrangement. 



* By orientation is implied the directions in space, relatively to some given 

 directions, of any axes (whether crystallographic, magnetic, acoustic, optic, thermic, or 

 any other). Thus M ah c indicate" the order of magnitude of the parameters of a tri- 

 metric crystal, say one of aragonite, and these are fixed in position, a, for instance, 

 being vertical, b horizontal, running from right to left, and c hori- 

 zontal, running from front to back, the magnetic orientation of such 

 a crystal is found to be such that the line c has the strongest tendency 

 —a. to stand equatorially. Aragonite is a diamagnetic substance, and 

 this, together with its orientation, is designated by Professor 

 Grailich by the symbol ^ (c a b) which serves to indicate that the 

 lines of greatest, mean, and least magnetic action are in the order 

 c ah, the greatest crystallographic axis being that of least magnetic action Tand 

 therefore in this diamagnetic substance also that of greatest molecular density ), 

 while the mean and least crystallographic axes are respectively the least and the 

 mean of the magne -crystal line axes. The magnetic orientation of the mineral 

 staurolite, which is a para-magnetic substance, is expressed by tj- (a c b), so that 

 here the same axis is the direction at once of the greatest crystallographic 

 parameter and of the greatest magnetic action, i.e., the axis of greatest density. 



/I 



