POTASSIUM, RUBIDIUM, CESIUM, AND LITHIUM 579 



ments replace hydrogen atom for atom. Chlorine' is able to take the 

 place of hydrogen by metalepsis, and the alkali metals take the placo 

 of hydrogen in water and acids. As it is possible to consecutively re- 

 place every equivalent of hydrogen in a hydrocarbon by chlorine, so it 

 is possible in an acid containing several equivalents of hydrogen to 

 replace the hydrogen consecutively equivalent after equivalent by 

 an alkali metal ; hence an atom of these elements is analogous to an 

 atom of hydrogen, which is taken, in all cases, as the unit for the 

 comparison of the other elements. In ammonia, and in water, chlorine 

 and sodium are able to bring about a direct replacement. According 

 to the law of substitution, the formation of sodium chloride, NaCl, 

 at once shows the equivalence of the atoms of the alkali metals and the 

 halogens. The halogens and hydrogen and the alkali metals combine 

 with such elements as oxygen, and it is easily proved that in such com- 

 pounds one atom of oxygen is able to retain two atoms of the halogens, 

 of hydrogen, and of the alkali metals. For this purpose it is enough to 

 compare the compounds KHO, K 2 O, HC10, and C1 2 0, with water. It 

 must not be forgotten, however, that the halogens give, with oxygen, 

 besides compounds of 4;he type R 2 O, higher acid grades of oxidation, 

 which the alkali metals and hydrogen are not capable of forming. We 

 shall soon see that these relations are also subject to a special law, 

 showing a gradual transition of the properties of the elements from 

 the alkali metals to the halogens. 43 



The atomic weights of the alkali metals, lithium 7, sodium 23, 

 potassium 39, rubidium 85, and caesium 133, show that here, as in the 

 class of halogens, the elements may be arranged according to their 

 atomic weights in order to compare the properties of the analogous 

 compounds of the members of this group. Thus, for example, the 

 platinochlorides of lithium and sodium are soluble in water ; those 



15 We may here observe that the halogens, and especially iodine, may play the part 

 of metals (hence iodine is more easily replaced by metals than the o'ther halogens, and it 

 approaches nearer to the metals in its physical properties than the other halogens). 

 Schiitzenberger obtained a compound C^H^OtOCl), which he called chlorine acetate, by 

 acting on acetic anhydride, (C 2 H 5 O) 2 O, with chlorine monoxide, C1 2 O. With iodine this 

 compound gives off chlorine and forms iodine acetate, C 2 H 3 O(OI), which also is formed 

 by the action of iodine chloride on sodium acetate, C2H 3 O(ONa). These compounds are 

 evidently nothing else than mixed anhydrides of hypochlorous and hypoiodous acids, or 

 the products of the substitution of hydrogen in BHO by a halogen (see Chapter XI., 

 Notes 29 and 78 bis). Such compounds are very unstable, decompose with an explosion 

 when heated, and are changed by the action of water and of many oiher reagents, which 

 is in accordance with the fact that they contain very closely allied elements, as does C1 2 O 

 itself, or IC1 or KNa. By the action of chlorine monoxide on a mixture of iodine and 

 acetic anhydride, Schiitzenberger also obtained the compound I(C 2 H 3 2 )3, which is 

 analogous to IC1 5 , because the group C 2 H 5 O 2 is, like Cl, a halogen, forming salts with 

 the metals. Similar properties are found in iodospbenzene (Chapter XL, Note 79), 



