involved in the Construction of Artillery. 353 



Note E. — (Sect. 15.) 



The law of crystalline arrangement in crystallizable masses, announced in the text, although 

 passing as a clue of illustration throughout this Treatise, would demand a separate work, to 

 treat it as its important relations to many of the most interesting questions of Physics, and 

 its enunciation, for the first time, would deserve. 



To fulfil this in a Note is impossible; a few remarks should, however, be added to the 

 text. By principal axis of the integrant crystal, in metallic masses in the act of becoming 

 crystallomotrically arranged, under the influence of heat, or of pressure due to its motion, 1 

 do i»ot necessarily mean the optic axis, were such determinable for opaque bodies such as the 

 metals, but that symmetric axis of the integrant crystal, whose position, after consolidation of 

 the mass, is found coincident with the direction in which the heat wave has passed out from the 

 mass, if cooling, or into the mass, if heating; and which direction is necessarily that of least 

 pressure within the mass, the pressure being that due to distortion or change of form, by 

 contraction or expansion. As matter of observation, this is found to be the longest axis of 

 the crystal in metals, and perhaps in all other crystalline bodies. 



But although not ascertainably the optic axis in metallic crystal?, future investigation 

 will most probably show, through such transparent bodies as assume in mass the same 

 crystallometric arrangement, that the principal axis, as the term is used by me, has direct 

 relations to the optic axis. 



It is certain that it must have immediate relations to the axes of elasticity of Fresnel, 

 which again are in direct connexion with the optic axes, as Savart has shown. The 

 relation of these latter, to local or unequal pressui'e within the mass, have already formed 

 the subject of masterly investigations by Seebeck, Biot, Fresnel, Brewster, and others ; 

 and the analogies betweeen the optic effects due to pressure and to change of temperature 

 in transparent solids transmitting polarized light, have been lucidly pointed out by Sir 

 John Herschell, who has well explained that in heating or cooling masses, such internal 

 strains or pressures are produced by expansion or contraction as reduce the proximate 

 cause of the phenomena simply to one of pressure ; heat itself having (so far as inferable 

 from those facts) no specific action on the crystalline arrangement, but being merely the 

 means through which internal and unequal pressures are produced. Mitcherlich's facts, 

 long since ascertained, as to the unequal expansion of crystals of certain systems in diffe- 

 rent axes, even when uniformly heated, indicate unequal elasticity in their respective axes, 

 as well as unequal resistance to the transmission of the heat wave; the latter fact — the 

 inequality of conducting power of crystalline bodies in different axes — is sustained by the 

 researches of Senarmont ("Annal. de Chim.," 3 ser. xxi. 457, xxii. 179, xxviii. 279) in 

 papers of the highest interest. He even found that unequal pressure, in homogeneous 

 uncrystallized solids, altered their conducting power in different directions. 



