A Sketch of the Laws of Chemical Combination. 315 



quaternary combinations formed by the substances before mentioned ; and 

 they are susceptible of a very simple classification. 



I. The oxy-salts. In all of these, with the exception of the compounds 

 of ammonia, both acids and bases contain oxygen. They may be typified 

 by sulphate of potash. 



II. The hydro-salts. This order is limited by Dr. Turner to those salts 

 in which both the acid and the base contain hydrogen, and in which oxygen 

 does not exist. The only examples of this kind of composition are the 

 combinations of the hydracids with ammonia, and with phosphuretted iiy- 

 drogen. 



III. The sulphur-salts. The composition of these has been already ex- 

 plained. 



IV. The haloid salts. These are analogous in composition to sulphur 

 salts, being formed by the union of the chlorides, iodides, bromides, fluor- 

 ides, and cyanurets, of different elementary substances with each other.* 

 Their composition may be illustrated by that of the ferro-cyanuret of 

 potassium (commonly called prussiate of potash), which consists of cyan- 

 uret of iron, as a haloid acid, combined with cyanuret of potassium, which 

 acts as its base. Under this class some very curious combinations occur, 

 in which it appears that substances having most decided acid properties, 

 such as the hydriodic, hydrosulphuric, hydrocyanic, and even the hydro- 

 chloric (muriatic) acids, act as electropositive elements or bases to the 

 chlorides, iodides, &c. of platinum and other metals. 



Not only do the combinations of oxygen, cyanogen, &c. with different 

 bodies, unite to form salts, but even their different cnuibinations with the 

 same body. Thus, chromic acid (1 chromium, 3 oxygen) will unite with 

 the sesquioxide of chromium (1 chr. 1^ ox.) ; tungstic acid with oxide of 

 tungsten ; sesquioxide of iron with the protoxide, to form the black oxide ; 

 and sesquicyanuret of iron with the protocyanuret, to form prussian blue. 

 It is only very recently that the composition of the latter substance has 

 been satisfactorily ascertained. In like manner, the oxide of a metal will 

 occasionally unite with its chloride, iodide, &c. forming oxy-chlorides, 

 oxy-iodides, &c. Interesting examples of such compounds are afforded by 

 the pigment known as Brunswick green, which is an oxy-chloride of cop- 

 per, — the same substance which is generated during the corrosion of 

 copper in sea water; and by the mineral or patent yellow, which is an 

 oxy-chloride of lead. 



Some of the elementary substances, and a great proportion of the various 

 orders of compounds, are usually obtained in a crystalline form. The 



• The term haloid salts is here employed in the sense in which it is used by Bons- 

 dorff fAnnales do Chimie, xliv. 18'J), and copied from him by Dr. Turner (Elements 

 of Chemistry, p. 6Gy), differing from its application by Berzelius, who includes 

 under it the simple or bi-clcmentary chlorides, iodides, &c. 



