488 PRINCIPLES OF CHEMISTRY 



peroxides have also recently become known in the case of chromium, sulphur, 

 titanium, and many other elements, and I have sometimes heard it said that 

 discoveries of this kind weaken the conclusions of the periodic law in so far 

 as it concerns the oxides. I do not think so in the least, and I may remark, 

 in the first place, that all these peroxides are endowed with certain properties 

 obviously common to all of them, which distinguish them from the actual, 

 Higher, salt-forming oxides, especially their easy decomposition by means of 

 simple contact agencies ; their incapability of forming salts of the common 

 type ; and their capability of combining with other peroxides (like the faculty 

 which hydrogen peroxide possesses of combining with barium peroxide, dis- 

 covered by Schoene). Again, we remark that some groups are especially 

 characterised by their capacity of generating peroxides. Such is, for instance, 

 the case in the sixth group, where we find the well-known peroxides of 

 sulphur, chromium, and uranium ; so that further investigation of peroxides 

 will probably establish a new periodic function, foreshadowing that molyb- 

 denum and tungsten will assume peroxide forms with comparative readiness. 

 To appreciate the constitution of such peroxides, it is enough to notice that 

 the peroxide form of sulphur (so-called persulphuric acid) stands in the same 

 relation to sulphuric acid as hydrogen peroxide stands to water : 



H(OH), or ItjO, responds to (OH)(OH), or H 2 2 , 

 and so also 



H(HSOJ, or H 2 S0 4 , responds to (HS0 4 )(HS0 4 ), or H. 2 S 2 8 . 



Similar relations are seen everywhere, and they correspond to the principle 

 of substitutions which I long since endeavoured, to represent as one of the 

 chemical generalisations called into life by the periodic law. So also 

 sulphuric acid, if considered with reference .to hydroxyl, and represented as 

 follows 



HO(S0 2 OH), 



has its corresponding compound in dithionio acid 

 (S0 2 OH)(S0 2 OH), or H 2 S 2 6 . 



Therefore, also, phosphoric acid, HO(POH 2 S ), has, in the same sense, its 

 corresponding compound in the subphosphoric acid of Saltzer : 



), or H 4 P a 6 ; 



and we must suppose that the peroxide compound corresponding to phosphoric 

 acid, if it be discovered, will have the following structure : 



(H.,P0 4 ) 2 or H 4 P 2 8 = 2H.O + 2P0 3 . 13 

 So far as is known at present, the highest form of peroxides is met with in 



is Jn this sense, oxalic acid, (COOH) 2 , also corresponds to carbonic acid, OH(COOH), 

 in the same way that ditliionic acid corresponds to sulphuric acid, and subphosphoric 

 acid to phosphoric; hence, if a peroxide corresponding to carbonic acid be obtained, 

 .it will have the structure of (HCO 3 ) 2 , or H 2 C 2 6 =H 2 O + C 8 6 , So .also lead must have 

 a real peroxide, Pb 2 O 6 . 



