COM P< ) I" N I )S o K NITROGEN WITH HYDROGEN AND QXYG K X 275 



temperature are thus made quite clear, and as at the boiling point a 

 density 38 was obtained, therefore at that temperature the vapours 

 consist of a mixture of 79 parts by weight of N 2 O 4 with 21 parts by 

 weight of ISTO 2 . 45 It is evident that a decomposition here takes place 

 whose peculiarity consists in the fact that the product of decomposition, 

 NO 2 , is polymerised (i.e. becomes denser, combines with itself) at a 

 lower temperature ; that is, the reaction, 



N 2 O 4 =NO 



is a reversible reaction, and consequently the whole phenomenon repre- 

 sents a dissociation in a homogeneous gaseous medium, where the 

 original substance, N 2 O 4 , and the resultant, NO 2 , are both gases. The 

 measure of dissociation will be expressed if we find the proportion of the 

 quantity of the substance decomposed to the whole amount of the sub- 

 stance. At the boiling point, therefore, the measure of the decomposi- 

 tion of nitrogen peroxide will be 21/(79 + 21)=O21, or 21 p.c. ; at 

 135 it=l, and at 10 it =0 that is, the N 2 O 4 is not then de- 

 composable. Consequently here the limits of dissociation are 10 and 

 135 at the atmospheric pressure. 46 Within the limits of these tem- 

 peratures the vapours of nitrogen peroxide have not a constant density, 

 and above and below these limits definite substances exist. Thus 

 above 135 N 2 O 4 has ceased to exist and NO 2 alone remains. It is 



45 Because if x equal the amount by weight of N. 2 O 4 , its volume will = x/46, and the 

 amount of NO 3 will =100 x, and consequently its volume will =(100 x), 23. But the 

 mixture, having a density 38, will weigh 100, consequently its volume will =100/38. 

 Hence Z/46 + (100 -*)/23 = 100/86, or x = 79'0. 



46 The phenomena and laws of dissociation, considered by us in only separate and 

 particular instances, are discussed in detail in works on theoretical chemistry. Besides, 

 certain points in the doctrine of chemical equilibria are still subject to some doubt owing 

 to the recent date at which the exact study of this subject commenced. Nevertheless, 

 in respect to nitrogen peroxide, as an historically important example of dissociation in a 

 homogeneous gaseous medium, we will cite the results of the careful investigations 

 (1885-1886) of E. and L. Natanson, who determined the densities under variations of 

 temperature and pressure. The measure of dissociation, expressed as above (it may also 

 be expressed otherwise for example, by the ratio of the substance decomposed to that 

 unaltered), proves to increase at all temperatures as the pressure diminishes, which 

 would be expected for a homogeneous gaseous medium, as a decreasing pressure aids 

 the formation of the lightest product of dissociation (that having the least density or 

 largest volume). Thus, in Natansons' experiments the measure of dissociation at in- 

 creases from 10 p.c. to 30 p.c., with a decrease of pressure of from 251 to 38 mm. ; at 49'7 it 

 increases from 49 p.c. to 93 p.c., with a fall of pressure of from 498 to 27 mm., and at 100 it 

 increases from 89'2 p.c. to 99'7 p.c., with a fall of pressure of from 732'5 to 11'7 mm. At 

 180 and 150 the decomposition is complete that is, NO 2 only remains at the low pres- 

 sures (less than the atmospheric) at which the Natansons made their determinations ; 

 but it is probable that at considerable pressure (of several atmospheres) molecules of 

 N 2 O 4 would still be formed, and it would be exceedingly interesting to trace the pheno- 

 mena under the conditions of both very considerable pressures and of relatively large 

 volumes. 



T 2 



