316 Intelligence and Miscellaneous Articles. 



being charged with anhydrous chloride of calcium previously satu- 

 rated with the ammonia yapour. 



Two hundred and fifty substances were submitted to contact with 

 the liquid, and the general results in each case recorded. The only- 

 elementary substances soluble in it were the alkali-metals proper, also 

 iodine (bromine was not tried), sulphur, and phosphorus. The more 

 frequently soluble inorganic salts were nitrates, chlorides, bromides, 

 and iodides ; whilst oxides, fluorides, carbonates, sulphides, and sul- 

 phates were very generally insoluble. Many saline substances, espe- 

 cially certain chlorides, bromides, iodides, and sulphates, absorbed 

 ammonia freely, and swelled greatly, but did not dissolve. The beha- 

 viour of the chlorides of mercury was peculiar. 



Various compounds of carbon were submitted to the action of the 

 solution of potassium in the liquefied vapour ; the free potassium 

 disappeared, but no elementary carbon was liberated. 



" On the Law of Extraordinary Refraction in Iceland Spar." Bv 

 G. G. Stokes, M, A. Sec. R.S. 



It is now some years since I carried out, in the case of Iceland 

 spar, the method of examination of the law of refraction which I 

 described in my report on Double Refraction, published in the 

 Report of the British Association for the year 1 862, p. 2/2. A prism, 

 approximately right-angled isosceles, was cut in such a direction as 

 to admit of scrutiny, across the two acute angles, in directions of the 

 wave-normal within the crystal comprising respectively inclinations 

 of 90° and 45° to the axis. The directions of the cut faces were 

 referred by reflection to the cleavage-planes, and thereby to the axis. 

 The light observed was the bright D of a soda-flame. 



The result obtained was, that Huygens's construction gives the 

 true law of double refraction within the limits of errors of observation. 

 The error, if any, could hardly exceed a unit in the fourth place of 

 decimals of the index or reciprocal of the wave-velocity, the velocity 

 in air being taken as unity. This result is sufficient absolutely to 

 disprove the law resulting from the theory which makes double 

 refraction depend on a difl'erence of inertia in different directions. 



I intend to present to the Royal Society a detailed account of the 

 observations ; but in the mean time the publication of this prelimi- 

 nary notice of the result obtained may possibly be useful to those 

 engaged in the theory of double refraction. 



XXXVIII. Intelligence and Miscellaneous Articles. 



REPORT ON A MEMOIR BY MM. E. LUCAS AND A. C;\ZIN, ON THE 

 DURATION OF THE ELECTRIC SPARK. BY EDM. BECQUEREL"^. 



IX/TM. F. LUCAS and A. Cazin purposed to estimate the duration 

 ^■^ of electric sparks in some determinate circumstances, especially 

 when the dimensions of the exciting batteries are changed, and the 



* The experiments were repeated before the members of the Commis- 

 sion (MM. Morin, Le Vemer, Fizeau, Jamin, and the Reporter, Edm. 

 Becquerel), at the Conservatoire des Arts et Metiers. 



