804 PRINCIPLES OF CHEMISTRY 



halogens in one group, the periodic system of the elements only requires 

 that it should bear an analogy to the halogens in the higher type of 

 oxidation i.e. in the salts and acids whilst it requires that as great 

 a difference should be expected in the lower types and elements as there 

 exists between chromium or molybdenum and sulphur or selenium. 

 And this is actually the case. The elements of the seventh group form 

 a higher salt-forming oxide, R 2 O 7 , and its corresponding hydrate, 

 HR0 4 , and salts for example, KC1O 4 . Manganese in the form of 

 potassium permanganate, KMnO 4 , actually presents a great analogy in 

 many respects to potassium perchlorate, KC1O 4 . The analogy of the 

 crystalline form of both salts was shown by Mitscherlich. The salts of 

 permanganic acid are also nearly all soluble in water, like those of 

 perchloric acid, and if the silver salt of the latter, AgClO 4 , be sparingly 

 soluble in water, so also is silver permanganate, AgMn0 4 . The specific 

 volume of potassium perchlorate is equal to 55, because its specific 

 gravity =2-54 ; the specific volume of potassium permanganate is equal 

 to 58, because its specific gravity =2-71. So that the volumes of 

 equivalent quantities are in this instance approximately the same 

 whilst the atomic volumes of chlorine (35-5/1-3 = 27) and manganese 

 (55/7-5) are in the ratio 4 : 1. In a free state the higher acids HC10 4 

 and HMn0 4 are both soluble in water and volatile, both are powerful 

 oxidisers in a word, their analogy is still closer than that of chromic 

 and sulphuric acids, and those points of distinction which they present 

 also appear among the nearest analogues for example, in sulphuric and 

 telluric acids, in hydrochloric and hydriodic acids, &c. Besides Mn 2 O 7 

 manganese gives a lower grade of oxidation, Mn0 3 , analogous to 

 sulphuric and chromic trioxides, and with it corresponds potassium 

 manganate, K 2 MnO 4 , isomorphous with potassium sulphate. 16 In the 

 still lower grades of oxidation, Mn 2 O 3 and MnO, there is hardly any 

 similarity to chlorine, whilst every point of resemblance disappears 

 when we come to the elements themselves i.e. to manganese and 

 chlorine for manganese is a metal, like iron, which combines directly 

 with chlorine to form a satine compound, MnCl 2 , analogous to magne- 

 sium chloride. 17 



Manganese belongs to the number of metals widely distributed in 



16 The comparison of potassium permanganate with potassium perchlorate, or of 

 potassium manganate with potassium sulphate, shows directly that many of the physical 

 and chemical propertieo of substances do not depend on the nature of the elements,, 

 but on the atomic types in which they appear, on the kind of movements, or on the posi- 

 tions in which the atoms forming the molecule occur. 



17 If, however, we compare the spectra (Vol. L p. 665) of chlorine, bromine, and 

 iodine with that of manganese, a certain resemblance or analogy is to be found connect- 

 ing manganese both to iron and to chlorine, bromine, and iodine. 



