1825.] 



Three new Salts of Soda, 



437 



The primary form of common sulphate 

 of soda is a doubly obhque four-sided 

 prism with the following angles. 



MonT 108° 



P on T 101 30' 



P on M 128 (by a common goniometer) 



The acute edges of the prism are fre- 

 quently truncated, making the prism six- 

 sided. The crystals of the sesquisulphate M 

 of soda when first formed are perfectly 

 transparent ; but I have never been able 

 to obtain them in a state fit for measure- 

 ment. 



I believe the primary form 

 to be a four-sided right prism. 

 The only forms which I have 

 observed are represented in 

 figures A and B, A repre- 

 sents an eight-sided prism 

 terminated by a four- sided 

 pyramid, having a rhomb P 

 instead of its apex. The in- 

 clination of M on T was 90°. 

 Hence I ccmceive M and T 

 to be two primary faces of 

 the original four-sided right 

 prism. The other faces, c/, e, are the truncation of the edges"of 

 the prism. P, I consider as the remains of the primary terminal 

 tion of the prism. Its position is oblique, though I have not 

 been able to measure the angle very exactly. The four pyrami- 

 dal faces, fif, by c, may be formed by decrements on the terminal 

 angles of the primary prism. I have not met with any crystal 

 exactly similar to figure B. The lateral edges of the primary 

 prism are always truncated, but the bihedralsiuninit represented 

 in the figure occurs occasionally. It is obviously produced by 

 a decrement on two of the edges of the base of the primary 

 prism. Most commonly the two sets of decrements represented 

 in these two figures occur together, giving the terminal truncated 

 pyramidal figure seven faces instead of five. 



The taste of the salt is very acid. When a crystal was laid 

 upon blotting paper, the paper became moist and acid, and con- 

 tinued so ; yet the crystal did not apparently absorb moisture ; 

 but continued hard and firm, and quite dry on the surface. 

 Indeed the paper remained dry, if the crystals had been washed 

 in water. There was not the least tendency to effloresce, though 

 the salt was exposed for several days to the air during dry 

 weather. 



