178 Abstract of recent Researches on the quantity of 

 The symbols of the simple forms are,— 



P 

 m 



{100}, 

 {010}, 

 {001}, 

 {101}, 



/ 



{110}, 



{111}, 



{112}, 

 {113}. 



The optic axes lie in a plane parallel to the face m, and 

 make angles of 21° 46' with a normal to p. The ratios of the 

 velocity of light in air to its velocity within the crystal are 

 1-571, 1'576, 1-614 for rays in planes parallel to t, », pi 

 polarized in those planes respectively. 



St. John's College, July 30, 1841. W. H. MlLLER. 



XXVIII. Abstract of recent Researches on the quantity of Heat 

 evolved in Chemical Combination^ particularly those of MM. 

 Dulong, Hess and Andrews. 



[Continued from p. 25, and concluded.] 



rj^HE law described at the conclusion of the former portion 

 A of this abstract was founded on the following evidence, 

 the apparatus used by M. Hess being the same employed in 

 the experiments by which the first law was found. Sulphuric 

 acid of various degrees of concentration, but of definite atomic 

 constitution, was mixed with a slight excess of water of am- 

 monia (sp. gr. 0-935), and the following results obtained : — 



Oil of vitriol H O 4- S O s gave 595*8 units of caloric. 



Acid of... 2HO + SO a ... 518-9 

 3 HO + SO a ... 480-5 



6HO+SO, 



446-2 



As the experiment was not made with anhydrous acid, it 

 need not be taken into account, and therefore commencing 

 with oil of vitriol, and adding to the quantity of heat evolved 

 by the ammonia in combining with each dilute acid, the quan- 

 tity of heat which had been previously evolved by the oil of 

 vitriol, therein, to form the dilute acid, the following numbers 

 are obtained : — 



