^4 Description of a refieciive Goniometer, 



tufts made their appearance, and an abundant deposition of 

 saline matter was formed. I found also, that if the solu- 

 tioa had been more concentrated, the precipitation would 

 have immediately taken place. 



On making a comparative experiment with arsenic acid, 

 I found that it forms a triple salt with ammonia and mag- 

 nesia, analogous to the phosphoric salt described by Dr. 

 Wollaston. The figure of the arsenical salt, as far as I could 

 determine it from a confused crystallization, is a trihedral 

 prism. 



We are therefore, I think, authorized from the experi- 

 ments herein detailed, to conclude, that the fossil which is 

 the subject of this paper is an arseniate of lead, and that, 

 if we state that the relative proportion of the constituent 

 parts of it is in one hundred, as follows, we shall not be 

 far from the truth : 



Oxide of lead - 6976 



Arsenic acid - 26*40 



Muriatic acid - 1'5S 



The silica and the oxide of iron, which account for a 

 portion of the loss, and the alumina and copper which are 

 sometimes found in an analysis of this fossil, 1 do not con- 

 ceive to be essential to it. 



The existence of a minute portion of muriatic acid as a 

 constant ingredient of it, is a curious fact: and it is still 

 more curious, when we consider it in connexion with the 

 analogy that, in this particular, it maintains with the natural 

 phosphates of lead. 



XV. Description, of a reflective Goniometer, By William 

 HyDE Wollaston, M.D,, Sec, i?.S.* 



X* ROM the advances that have been made of late years In 

 crystallography, a very large proportion of mineral sub- 

 stances may now be recognized, if we can ascertain the 

 angular dimensions of their external forms, or the relative 

 position of those surfaces that are exposed by fracture. But 

 though the modifications of tetrahedrons, of cubes, and of 

 those other regular solids, to which the adventitious aid of 

 geometry could be correctly applied, have been determined 

 with the utmost precision, yet it has been often a subject 

 of regret, that our instrurrjLUts for measuring the angles of 

 crystals are not possessed of equal accuracy, and that Iii 

 applying the goniometer to small crystals, where the radius 



«^- From Philosophical Traa»actions for 1809, Part II. 



IB 



