34 On the Chemical Constitution of Harmotome. 



A few minor modifications of both forms also occur, which it is 



unnecessary to notice. 



I was farther confirmed in these views of the connexion be- 

 tween the two forms, by the opportunity which the liberality 

 of Mr Allan afforded me of consulting the interesting catalogue 

 of his collection, drawn up by Mr Haidinger, in which I found 

 a series of figures of harmotome crystals, presenting, in so far 

 as I could judge, a transition of the one form into the other, 

 with some of those lesser modifications to which I have alluded. 



As the angle of the faces B, replacing the opposite edges of 

 the pyramid in Fig. 1, has been stated by Mr Phillips as 110° 26', 

 this will of course become the measure of the rhombic prism, 

 Fig. 3, if the foregoing views of the relation between the two 

 forms are correct. 



Before the connexion of the two forms Figs. 1 and 3 had oc- 

 curred to me, which was not until I had observed a crystal of 

 the. form Fig. 2, I commenced an analysis of a portion of the 

 crystals of the rhombic form. Fig. 3, under the idea that they 

 might present some modification of the usual constitution of 

 this mineral ; and although they proved to be merely a barytic 

 harmotome, yet the analysis seems to throw some little addi- 

 tional light on the connexion between the barytic and lime va- 

 rieties of the mineral. The steps of the analysis were as follows. 



(a.) 7.37 grains of the crystals in coarse powder lost, by ig- 

 nition, 1.1 grain, equivalent to a loss of 14.9~5 per cent. 



ijb,) 16.07 grains of the crystals which had been previously 

 treated with acidulated water, to remove all adhering calcareous 

 spar, were reduced to impalpable powder, and then left for three 

 or four days in contact with muriatic acid, a moderate heat being 

 occasionally applied. The mass did not gelatinize ; but, as will 

 afterwards appear, the action was quite sufficient for the purpose 

 of analysis. I'he whole was evaporated to dryness. A little 

 muriatic acid was then poured over it, and left for some hours, 

 when water was added, and heat applied. The silica was then 

 separated by filtration. After ignition, it weighed 7.55 grains. 

 It dissolved in boiling caustic potash ley, except a residue of 

 .47, which was resolved by fusion with carbonated alkalies, and 

 other necessary steps, into .4 o. silica, and .01 of oxide of iron. 

 The total silica thus amounts to 7.48 grains. 



