556 PRINCIPLES OF CHEMISTRY 



"mixed with it. 15 When thrown on to 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 oxidises the charcoal, in consequence of 

 which potassium carbonate and carbonic anhydride are formed : 

 4KNO 3 + 50 a= 2K 2 C0 3 + 3CO 2 + 2N 2 . This phenomenon depends on 

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

 it do'es 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 extern al heat. A similar oxidation or combustion at 

 the expense of the contained oxygen takes place when nitre is heated 

 with -different combustible substances If a mixture of sulphur and 

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

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

 nitrogen of the nitre is evolved as gas 2KNO 3 -f2S = K 2 SO 4 4-Na 

 4-S0 2 . A similar phenomenon -occurs when nitre is heated with 

 many metals. The oxidation of those metals which are able to form 

 acid oxides with an excess of oxygen is especially remarkable. In 

 this case they remain in combination with potassium oxide as potassium 

 Salts. Manganese, antimony, arsenic, iron, chromium, &c. are in- 

 stances of this. kind. These elements, like carbon and sulphur, displace 

 free nitrogen. The lower oxides of these metals when fused with nitre 

 pass into the higher oxides. Organic substances are also oxidised 

 when heated with nitre that is, they burn at the expense of the nitre. 

 It will be readily understood from this that nitre is frequently used in 

 practical chemistry and the arts as an oxidising agent at high temper- 



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

 nitrate being 85, and of potassium nitrate 84, parts in 100 parts of water, then-* 



p~ 10 20 80 40 50 60 70 80 90 



<5=110 186 186 188 106 81 78 54 41 



The maximum solubility proved not to correspond with 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 in solution a third substance, 

 namely water, plays a part, although an attraction between the salts, like that which 

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



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

 gen. The nitrite KNO 3 is first formed and then potassium oxide. The admixture of 

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

 Qxygen in this ease naturally passes over to the metal. 



