POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 547 



but at a hif/h temperature it acts as a powerful oxidising agent, 

 because it gives up a considerable amount of oxygen to substances 

 mixed with it. 1 "' When thrown on incandescent charcoal it brings 

 about its rapid combustion, and a mechanical mixture of powdered 

 charcoal and nitre ignites when brought into contact with a red-hot 

 substance, and continues to burn by itself. In this action, nitrogen is 

 evolved, and the oxygen goes to oxidise the charcoal, in consequence of 

 which potassium carbonate and carbonic anhydride are formed : 

 4KNO 3 + 5C=2K 2 CO 3 + 3CO. 2 + 2N 2 . This phenomenon depends on 

 the fact that oxygen in combining with carbon evolves more heat than 

 it does in combining with nitrogen. Hence when once the combustion 

 has been started at the expense of the nitre, it is able to go on without 

 requiring the aid of external heat. A like oxidation or combustion at 

 the expense of the contained oxygen proceeds when nitre is heated 

 with different combustible substances. If a mixture of sulphur and 

 nitre be thrown upon a red-hot surface, then the sulphur burns, form- 

 ing potassium sulphate and sulphurous anhydride. In this case, also, 

 the nitrogen of the nitre is evolved as gas : 2KNO 3 + 2S=K 2 SO 4 -fN 2 

 + SO. 2 . A similar phenomenon takes place when nitre is heated with 



and sodium nitrates. The first salt fuses at 339 and the second at 316, and if p be 

 the percentage amount of potassium nitrate, then the result obtained 



#=10 20 30 40 50 60 70 80 90 



298 288 268 242 231 231 242 284 306 



which confirms Shaffgotsch's observation (1857) that the lowest fusing point (about 231) 

 is given by mixing molecular quantities (^? = 54'3) of the salts that is, in the formation 

 of the alloy, KNO 3 ,NaNO 3 . 



A somewhat similar result was discovered by the same observers for the solubility of 

 mixtures of these salts at 20 in 100 parts of water. Thus, if p be the weight of potas- 

 sium nitrate mixed with 100^? parts by weight of sodium nitrate taken for solution, 

 and c be the quantity of the mixed salts which dissolves in 100, the solubility of sodium 

 nitrate being 85, and of potassium nitrate 34, parts in 100 parts of water, then 



p= 10 20 80 40 50 60 70 80 90 



C = 110 13(5 18(5 188 106 81 73 54 41 



The maximum solubility proved not to answer to the most fusible mixture, but to one 

 much richer in sodium nitrate. 



Both these phenomena show that in homogeneous liquid mixtures the chemical forces 

 that act between substances are the same as those that determine the molecular weights 

 of substances, even when the mixture consists of such analogous substances as potas- 

 sium and sodium nitrates, between which there is no direct chemical interchange. It is 

 instructive to note also that the maximum solubility does not correspond with the mini- 

 mum fusing point, which naturally depends on the fact that a third substance, namely 

 water, participates in solution, although an attraction between the salts, like that which 

 exists between sodium and potassium carbonates (Note 8), also partially acts. 



15 Fused nitre, with a further rise of temperature, disengages oxygen and then nitro- 

 gen. The nitrite KNO.^ is first formed and then potassium oxide. The admixture of 

 certain metals for example, of finely -divided copper aids the last decomposition. The 

 oxygen in this case naturally passes over to the metal. 



N N 2 



