574 Intelligence and Miscellaneous Articles. 



MAGNECRYSTALLIC PROPERTY OP CALCAREOUS SPAR. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, Glasgow College, Nov. 7, 1851. 



A mistake (I cannot call it a misprint) which occurred in the 

 footnote to § 12 of my paper on Magnetic Induction, published in 

 your Number for last March, has, although corrected in the " Errata" 

 of the volume containing it, caused considerable perplexity regarding 

 my meaning, as I perceive by some remarks of Dr.Tyndall's, contained 

 in a foot-note on his paper on the Polarity of Bismuth, published 

 in your Number for this month. Vou will oblige me by publishing 

 the following, which is the correct form of the passage referred to. 



" Thus, a ball cut out of a crystal of pure calcareous s})ar, which 

 tends to turn with its optic axis perpendicular to the lines offeree, and 

 which tends as a whole from places of stronger towards places of 

 weaker force, would experience this latter tendency Isss strongly 

 when the optic axis is perpendicular to the lines of force than when it 

 is parallel to them ; since, according to § 8 of the text, the crystal 

 must have the greatest inductive capacity, or (the language in the 

 text being strictly algebraic when negative quantities are concerned) 

 least capacity for diamagnetic induction, perpendicular to the optic 

 axis." 



In the passage, as originally published, the word ** more" occurred 

 in the place of ** less." The mistake was pointed out to me last 

 April by Professor Stokes, and I immediately requested you to cor- 

 rect it, which you accordingly did by an intimation in the " Errata." 

 When the perplexity occasioned by the mistake is removed, it is ob- 

 vious to any one reading the passage carefully, that the mistake itself 

 was only a slip of the pen, as at the conclusion of the sentence it is 

 asserted that a crystal of pure calcareous spar must have the " least 

 capacity for diamagnetic induction, perpendicular to the optic axis." 



This conclusion is verified by Dr. Tyndall, who describes experi- 

 ments, in a paper published in your September Number, by which it 

 appears that the diamagnetic inductive capacity of calcareous spar in 

 a direction parallel to the optic axis is to its diamagnetic inductive 

 capacity perpendicular to the optic axis as 57 to 51. 



I remain. Gentlemen, 



Your obedient Servant, 

 William Thomson. 



[We have also received a communication on this subject from 

 Mr. Tyndall, who in reference to a note received by him from Prof. 

 Thomson writes as follows : — "I have only to say that the facts are 

 precisely what they are here stated to be. Previous to writing the 

 remarks in question, I looked to the Errata, but not it seems with 

 sufficient attention, for Professor Thomson's correction escaped me. 

 Not only do our results agree in principle, but the same substance 

 and form of substance which Professor Thomson had referred to in 



