PART III.

Metallic Preparations.

495

clature of this division of preparations is defective.' Somemetals are capable of so high a degree of oxidizement as toacquire acid properties, which is so particularly the case with thewhite oxide of arsenic, that it is regarded as an acid by severalchemists. The activity of the oxides of metals on the animalsystem appears to be regulated, with a few exceptions, by thequantity of oxygen with which they are combined • andtherefore as Dr. Murray has justly observed, « when a proI7ur\l Preparation of any metallic oxide has once beenestabhshed, and pract.t.oners have become accustomed toTspowers and strength the process ought not to be varied orchanged, from the idea of some trivial improvement; 2 ™alteration of circumstances, apparently of little importantmay give rise to a very important change in the result. Andit is nearly demonstrable, that the oxides of a metal formed bvd^rent processes, as, for example, by a process conducted inthe humid way, or by one with the application of heat cannSbe precisely the same." a » cannot

Besides the above effects of oxidizement on metals it ren

tz the ^ c r ble of u f ing with adds ' - d fo^&soS;

salts. The Metallic Salts, therefore, are oxid P « £™k- ,

;te„ tfc d nd f y? the case ' wheth - - S P rK

prepared be dissolved in an acid, or whether the snlr l^fi y

duct of the direct solution of a metal i„ ™ ?£ In' h e ZTrcase the metal first gams oxygen either from a part of the Sitself, or from the water, or the air, which it decomposes" andthe oxide thus formed is then dissolved by the iSS *the acid. Theproperties of the metallic salts ZeZT^lby the previous degree of oxidizement of the metl ™Athis u a point, the fixing of which in pharm ace S ' ? rations is of the first practical importance; for, if ! n ffindefinite degrees of oxidizement die metallic oVd e comb twith acids, the resulting salts must vary in as nTnL ias exist between the malimum and minimum of T7- &deSIn the preparation of the metallic ^ZrffoT^" 1 ^'

Sag!* ^^ ^ ^Wone-etbliirdthough there is no instance of a directtmbiStmn ofceS

1 Dr. Thomson haj endeavoured to remedv thi« rf.c„. i •lorUe to signify the lowest degree of« ZIZm perorit^Tt™ 5 the te ™ /"»"tntoxxdt &c. the intermediate degrees. ' P hlghest ; and '«KM,

* System of Mat, Med. ii,253.