PEOFESSOE G. P. GIEDWOOD. 7 



heat, the solid in sohition will again reappear, sometimes without form, as the various gums, 

 albumen or gelatine, sometimes in geometric forms, in the latter case in crystals. Why 

 should these bodies assume the particular forms they do? This question has doubtless 

 been asTced by many, but where is the answer. May not these forms be due to the molecu- 

 lar vibrations in the molecule at the moment of solidification? And where is growth most 

 rapid ? It is found t)n examination to be at the solid angles and edges of these crystals. 

 This leads to the examination of vilu-ations and their effect upon solid substances. If a 

 plate of metal be made to vibrate and dusted with particles of sand, it will lie found that 

 the vibrating body divides itself into parts vi' doulile motion and parts of rest with no 

 apparent motion, along which the particles of saud are arranged or aggregated, and as the 

 plate may be made to vibrate with vibrations of diflerent lengths or amplitudes and ditler- 

 ent rapidity, the length must vary in relation to the size of the plate, and if it be examined 

 further the vibrati(jns will lie found to travel from end to end oi' the plate and then be 

 reflected back again. The same will be observed in a glass of water or of mercurv made to 

 vibrate. As the waves of vibration are passing from one side to the other and returning, 

 there must be points where the crests of two waves meet at the same moment, and again 

 others where tlie crest of one wave meets the hollow of another. Where two crests meet 

 douT)le movement results ; where crest and hollow meet, one counteracts the other and no 

 movement is observed, although the impulses pass this point ; thus producing parts of 

 double motion and parts of rest, at the parts of rest, deposition or aggregation of matter 

 takes place. 



Applying this, then, to the formation of crystals, may it not be suggested tliat the 

 shape of the solid matter is determined by the length of the vibrations as regards 

 the size and density of the particular molecule in the crystals ? Deposition is found 

 to be greatest or most active at the angles and edges, and in some, notalily the salts 

 of the haloid elements, hopper-shaped crystals are of common occurrence, looking like 

 skeletons of the cube, the shape of the perfect crystals. May not these edges and angles 

 be the points of rest where crest and hollows meet? Small crystals of these substances are 

 always perfect, but as they grow larger the deposit takes place at the edges and angles, 

 which are developed without an equal development of the faces of the crystals, or at the 

 extremities of the axes of symmetry. Again, some crystals are very prone to assume 

 modified forms. 



iSTow, our vibrating plate with its sand can easily be made to vibrate with a certain 

 sound — that is, with a certain number of vibrations — the sound emitted will be a low note, 

 but it can be made to emit a high note, due to increased number of vibrations. If these be 

 divisible into the area of the vibrating body exactly, the figure will be a regular geometric 

 one, but if not exactly dividing the plate, then there will be a variation of some kind, a 

 curve will be produced, and a similar curve will be produced at the corresponding opposite 

 side. 



So, if a crystal, as sometimes happens, be arrested in development at one side, a corre- 

 sponding arrest will be found at the correspoiuling opposite side. 



So that if a plane be developed instead of an edge, a similar plane will be devclojied on 

 tlie correspon<ling opposite side instead of the edge of the perfect crystal. These compound 

 forms are seldom seen in the minute crystals, but are common in the larger ones, and it often 

 happens that such compound forms only show themselves when tlie crystal has reached a 



