10 Messrs. J. Tyndall and H. Knoblauch on the 



other lists in vain. In one German work on physics we fin* 4 

 Blutlaugensalz set down as a negative crystal with one op 

 axis, but whether the red or yellow salt is meant, th 

 does not explain. We have examined the crystal i 

 and find it positive with two optic axes. The mi 

 stands perpendicular to the principal cleavage. Sut 4 

 with this line horizontal, on closing the circuit it sets use 

 equatorial. Another exception to the law under considers 

 tion is here exhibited. 



M. Pliicker recommends the magnet as a practical irea; 

 of determining whether a crystal is positive or negative ; th, 

 method being attended with the peculiar advantage thrt *. 

 can be applied in the case of opake crystals, where all 'he 

 ordinary methods fail. We find accordingly, in his last i e- 

 moir on this subject, metallic and other opake crystals v, 

 optical properties attributed to them. Antimony is nega 

 with one optic axis; bismuth and arsenic are positive w 

 one optic axis. The foregoing experiments demonstrate i 

 insecurity of the basis on which this classification rests. 



By looking back upon the results described, it will be set 

 that we have drawn from each respective class of crystals om 

 or more examples which disobey the law of M. Pliicker. Oi 

 positive crystals with one axis, we have quartz; of positive 

 crystals with two axes, we have heavy spar, ccelestine and 

 ferrocyanide of potassium. Of negative crystals with one 

 axis, we have carbonate of lime and iron, and several others; 

 of negative crystals with two axes, we have dichroite, sugar, 

 sulphate of zinc, and sulphate of magnesia. It is due how- 

 ever to M. Pliicker to state, that in a considerable number of 

 cases we have found the law confirmed. Tourmaline, idocrase, 

 beryl, Iceland spar, saltpetre, arragonite, and many others all 

 confirm it. Singularly enough, these are the very crystals 

 with which M. Pliicker has experimented. It is therefore 

 not to be wondered at, that he should be led by such a mass 

 of concurring evidence to pronounce his law general. Had 

 his experiments embraced a sufficient number of cases, they 

 would doubtless have led him to the same conclusion to which 

 ours have conducted. 



Mr. Faraday has devoted considerable time to the investi- 

 gation of this intricate subject. His most notable experi- 

 ments are those with bismuth, antimony, arsenic, sulphate of 

 iron, and sulphate of nickel, which experiments we have care- 

 fully repeated. 



Bismuth. — Crystals of bismuth we have ourselves prepared, 

 by melting the metal in a Hessian crucible, placed within a 

 larger-one and surrounded by fine sand. In this state it was 



