1859.] as investigated by Liyht. 99 



The passage of Electricity through crystals has been studied 

 [Wiedemaun, Senarmont, Knoblauch], and given general results indica- 

 tive of analogy with those obtained in respect to the transmission of 

 light and heat; while the remarkable development by heating or 

 cooling a crystal, of electric tensions on parts of it morphologically 

 polar to each other [analogue and antilogue poles in terminally-polar, 

 and also central-polar pyro-electricity — Riess and Rose, Karsten 

 Pasteur, &c.] indicates that here, as in magnetically polar sub- 

 stances, and as in the alterations in volume effected by change 

 of temperature, the powers that, so to say, reside in the crystal, and of 

 which its crystalline form is the outward expression, are potent to 

 modify the character and the amount of the tensions induced by the 

 natural forces, which we call electricity, magnetism, and heat, and 

 that those powers have thereby a direction or localisation imposed on 

 them in accordance with the crystalline structure. 



Heat, indeed, must be viewed in more than one modo operandi. 

 As radiatit heat ; in the influence of the crystal upon its forward pro- 

 pagation ; in its polarisation, absorption, or transmission by {i.e. the 

 diathermanence of) the crystal, we have to associate it intimately 

 with light, to view it in short, as light endowed with longer wave 

 length. 



As heat of temperature (intensity of thermic excitement) we must 

 follow up its action on crystals as an agent causing increase of volume, 

 and therewith inducing a series of concomitant results of the highest 

 interest. In this respect it affords one of the most instructive means 

 at our disposal for the examination of crystal structure. Upon this 

 Professor Grailich and Dr. Viktor Yon Lang have brought mathema- 

 tical analysis to bear ; and by showing that the increase of temperature, 

 while changing the relative values of the parameters of a crystal, never 

 alters the irrational character of those parameters, have given to 

 Mitscherlich's beautiful and well-known results a new significance, as 

 Von Lang has also done with Rudberg's investigation on the change 

 in the action of aragonite on light induced by a change of temperature. 

 They have shown that all which is symbolised by the indices (the 

 general symbol {h k I), in a crystal — its symmetry, and therefore its 

 system — remains unaltered : the lengths of the parameters may vary, 

 the inclination of a leaning axis in the oblique systems, may change by 

 change of temperature, but the principle which Grailich establishes 

 as the " Law of Conservation of Zones," remains presiding over the 

 general crystal form, so that it is impossible for a crystal, by the mere 

 agency of a changing temperature, to drift from one system into 

 another.* 



while the mean and least parameters are the directions respectively of the least 

 and mean magne-crystalline set, 



* Thermic axes are those directions in a crystal along which it is altered, by 

 change of temperature, only in linear dimensiuns They are are fixed crystallo- 

 graphic directions. Every other crystallographic line that can be drawn through 

 the crystal^ changes not in length only, but also in direction relatively to these 

 axes. 



II 2 



