I9i> 



ON THE ANHYDROUS SULPHATE OF LIME. 



ftances. 



Arfeniates of 

 copper. 



Time proportions, it cannot be doubted but that the prefence 



of the water totally changes the nature of the combination of 



acid and earth when it comes to be joined with it, which, it 



appears to me, can only take place (b far as this water becomes 



itfelf an eflential component part of theftone. 



Remarks In This is not the only fubftance which enables us to afcertain 



proof that water that the water which formerly was confidered as a part fo- 



is eflential to ' • , • , i« 



the compofition reign to the ltones which contained it, becomes m reality, m 



any fub- feveral of them, an ingredient eflential to their nature. The 

 analyfis which Mr. Chenevix has made of the different fpecies 

 of arfeniates of copper, have prefented us a ftriking example 

 of this, efpecially in that variety of the third fpecies of my de- 

 fcription (fee Philofophical Tranfactions, 1801) to which I 

 have given the name of Hematiform. A movement of decom- 

 pofition, which is confined to the gradual lofs of their confti- 

 tuent water, totally changes the colour of thefe arfeniates, and 

 at laft completely difcolours them, at the fame time that it ren- 

 ders thofe that before had fome feeble tranfparency, perfectly 

 opaque. This lofs of the water always commences at the ex- 

 terior, and in this cafe the interior part preferves all its tranf- 

 parency as well as its colour, whiJft the exterior part is difco- 

 loured, and exhibits by the fhrinking of its furface, fometimes 

 to a very confiderable degree, fenfible marks of the lofs which 

 it has fuftained ; the water amounting to about one-fifth of its 

 mafs *. 



* In my defcription of the arfeniates of copper quoted above, I 

 have confidered this hematiform arfeniate, as well as thofe which I 

 have deiignated by the terms of indeterminate, capillary, and ami- 

 anthiform, only as being varieties of the fpecies in the acute octa- 

 hedral form •- the copper and the arfenical acid are in fact contained 

 in them in the lame proportions j but the water, which adds a new 

 conftituent part to them, and which did not exift in thofe varieties 

 of this third fpecies, which are in perfectly determined cryftals, 

 forms with them a real hydrate of this third fpecies. I therefore 

 think that it would be proper to feparate thefe varieties, in order to 

 form with them a fifth fpecies perfectly diftinct from the third. The 

 arfeniate of copper is one of the rood aftonifhing productions of 

 the mineral kingdom, by the immenfity of the afpects under which 

 it prefents itfelf, all which, neverthelels, have certainly a particular 

 caufe, which I am very far from pretending to have afcertained. 



In 



