^g2 LECTURE XXXIV. 



growing. If the two kinds of crystals come into direct contact, the rhombohedral 

 ones instantly become cloudy, tKe surface uneven, and prisms soon grow out 

 from many points of their margins. On contact with other solid bodies also, the 

 rhombohedra are transformed if still moist. If the liquid is in very thin films 

 and dries up around the rhombohedra before they are destroyed, the crystals retain 

 their form for weeks without efflorescing, and endure moderate pressure of foreign 

 bodies without change; but with stronger pressure or scratching, as well as on 

 mere contact with a prismatic crystal of saltpetre, they become transformed into 

 the prismatic form — a delicate cloud spreads over the surface, starting from the 

 point of contact, and they now behave to solid bodies like a heap of fine dust, 

 remaining at the same time transparent however.' 



I myself observed in a group of rhombohedra of potassium nitrate, in mutual 

 contact in a drop under the microscope, that when a prismatic crystal of the same 

 salt was pushed into the drop and came in contact with one of the rhombohedra, 

 the latter not only at once became cloudy, but the rhombohedra which were in 

 contact with one another were also progressively rendered cloudy. This clouding, 

 moreover, indicated the breaking up of the rhombohedron into innumerable small 

 prisms of the arragonite form. The most remarkable point in the matter is, 

 that it is only contact with saltpetre crystals of the arragonite form which is able to 

 call forth this alteration in the rhombohedric saltpetre crystals; for the quadratic 

 tablets of potassium ferrocyanide and the triclinic prisms of potassium chromate 

 may be in contact with the rhombohedral saltpetre crystals in a common mother- 

 liquor, without any change whatever occurring. 



These processes have one point in common with the phenomena of irritability 

 in organisms ; viz. that an entirely specific and at the same time mechanically 

 insignificant external influence produces an effect which corresponds to it neither 

 quantitatively nor qualitatively. And similarly also in many other cases. Thus 

 Gmelin says (loc, cit.), ' Mercuric iodide (Hgl^) crystallises at ordinary temperatures 

 in red quadratic tables ; on sublimation at a high temperature, on the contrary, in 

 yellow rhombic tables. The red crystals whenever they are heated become yellow, 

 and again turn red on cooling. The yellow crystals obtained by sublimation remain 

 unaltered on cooling, but on rubbing or contact the point touched turns red, and this 

 colour extends itself through the entire group of crystals, with a movement as if the 

 mass were animated. The external form of the yellow crystals is here maintained, 

 while the molecules take up the position of the other crystalline system. Each time 

 these are heated they become yellow, and again turn red on cooling.' 



Among numerous other examples I will only quote the behaviour of sulphur, 

 pientioned by Roscoe and Schorlemmer. If a hot solution of sulphur in alcohol, oil 

 of turpentine, or carbon disulphide is quickly cooled, there separate out first a few 

 monoclinic and then rhombic crystals. The rhombic ones correspond to the stable 

 equilibrium at ordinary temperatures, since Mitscherlich found that monoclinic sulphur 

 (which is formed on quickly cooling from a high temperature) slowly passes over at 

 ordinary temperatures into rhombic, the monoclinic crystal being transformed 

 into a group of rhoibibic crystals. This transformation proceeds more rapidly if the 

 monoclinic crystals are shaken or scraped, or exposed to the sun, and a definite 

 quantity of heat is set free in the process. • 



