^14 Analjjis of the Arfeniates 



If, upon that brown oxide, a fufficlent quantity of dilute 

 fulphuric acid is poured, it yields a blue fait, but in a pro- 

 portion greater by about 34 per cent, than if the green fub- 

 Itance had been employed. I imagine, therefore, that the 

 firfl: operation of this brown oxide is, to afl'unie the quantity 

 of water neceirary to confiitute a hvdrate; and, that the com- 

 bination of fulphuric acid takes place, not between the oxide, 

 but between the hydrate and that acid, to form a fait, which, 

 when cryftallized, has taken another portion of water in the 

 acl of cryftallization. It is a well known facf, that there is 

 a ftate ot concentration, when an aqueous folution of muriate 

 ot copper, gently heated, will change from a blueifh green 

 to a beautiful brown, which, upon coding, or by the affu- 

 fion of water, refumes its former tinge. This brown liquor 

 is probably a folution of muriate of copper; while the blue 

 liquor, like every green or blue folution of a cupreous fait, is 

 a. combination of the acid and the hydrate, or (as \\t fliould 

 fay in this cafe) a muriate of hydrate of copper. It is true, 

 I have not been able to produce, fo often as I could wifh, this 

 change of colour. I can, however, adduce the following in- 

 ftancc, as being much in favour of my opinion. 



It is evident that oxide of copper (for fo I fliall henceforth 

 call the brown fub fiance) has a very ftrong affinity for water j 

 becaufe the fixed alkalis, (unlefs when boiled upon it,) and 

 their carbonates, all of which eafily decompofe the falts of 

 copper, cannot difpoffefs the hydrate of copper of its water. 



This led me to imagine that I could perhaps, by fire, dif- 

 fipate not only the water of cryftallization, but that contained 

 in the hydrate j and leave the acid, if a potent one, ftill in 

 the fait. But I found that the affinity of the alkali, a6ling 

 upon the acid, had, in the humid way, determined an order 

 ot couibinatioii not to be efi'eclcd by heat ; for even fulphuric 

 acid was expelled before the water of the hydrate could be 

 completely diffipatcd. Upon refiefting on the fixity of acids, 

 I could find none fo proper for this experiment as the phof- 

 phonc. I therefore prepared fome artificial phofphate of 

 copper, by precipitating the nitrate of that metal by phof- 

 phate of foda. When waOicd and dried, it was in the form 

 of a fine bkieifli green powder, among which many crvftals 

 were difcernible almoft to the naked eye. 



One hvmdrcd parts of this, expofcd to a gentle red heat, 

 became of a much paler green, but pafled entirely to brown 

 when the temperature was fufficicnily elevated. I had then 

 a brown pholphale, not of hydrate, but of ox'de of copper, 

 and from which no acid had been volatilized. Its lofs of 

 weight was wholly from the water which had been expelled^ 



and 



