1825.] 



Mr, Brooke* $Repli/ to Dr. Brewster* 



331 



Fig. 2. 



But this insinuation is unfounded. It is not true that Dr. 

 Brewster had, as he asserts ^' distinctly described^' the combi- 

 nation in question in the paper he refers to. Nor, unless Dr. 

 Brewster has learned something more on the subject since he 

 wrote that paper, does he even now understand how the bypyra- 

 midal crystals are formed. 



The following is the short description I gave of this salt in 

 the Annals of Philosophi/ for January, 1824 : — 



Sulphate of Potash. 



The primary form of this salt was, I believe, first determined by Mr. Levy to be d, 

 right rhombic prism, and described in No. 30 of the 



Eoyal Institution Journal; but probably from not Fig. 1. 



possessing sufficiently expLmatory crystals, Mr. L. 

 has not pointed out the relation of its primary form 

 to the bi-pyramidal figure under which it generally 

 occurs. 



I have been enabled to do this in a very satisfac- 

 tory manner by means of a compound crystal which 

 I have obtained from the solution of a portion of this 

 salt in distilled water. 



Fig. 1 is a single modified crystal. 



3IonM- 120° SO' 



MonA 120 45 



Mone 146 22 



/tone 146 10 



cone" 112 20 



eone' 131 12 



Fig. 2 is the compound crystal, which consists of 

 three single crystals, so imited that their upper edges 

 meet at angles of 120", and consequently their planes 

 of junction inclme to each other at the same angle. 

 Hence 



MonM" 119° SO' 



eone" 130 24 



There is not in this brief notice any attempt to set myself up 

 as the discoverer of the composite character of the bypyramidal 

 crystals. On the cuntrary, I suppose that fact already known, 

 and the evident object of the notice was merely to point out the 

 precise relation of the simple to the compound crystal which had 

 not to my knowledge been previously ascertained. 



The reason why 1 did not refer to JDr. Brewster's paper on the 

 same subject was, that I knew it to be incorrect both in measure- 

 ment and description, and felt at that time no particular motive 

 to expose these inaccuracies, iTor should I have noticed them 

 now if the task had not been forced upon me by Dr. Brewster. 

 The prismatic planes which commonly appear on the simple 

 crystals of this salt are those marked c aiid c^^in fig. 1, the mutual 

 inclination of which is 112° 20' very nearly, but according to 

 Dr. Brewster it is 114° — an error of not much more than a 

 degree and a half, and which may, perhaps, not be an uncharitable 

 toeasure of Dr. Brewster's ordinary precision. 



