464 DR LYON PLAYFAIR AND MR J. A. WANKLYN ON A MODE OF 
The influence of the possible tension of aqueous vapour from strong solution 
of caustic soda does not seriously affect the calculation. If we reckon the gas 
absolutely dry, we get vapour-density of NO, = 2°69. 
If absolutely wet, we have vapour-density of NO, = 2°60. 
IV. 
Vapour-density at 4:2° C. 
Cubic Cent. at 0° C. 


and 760 m. nm. Grm. 
Vol. of N+ NO, 854-38 weighing -6711 
ae 294:15 ~, +3695 
60:23 ‘2016 
Therefore 60:23 C. C. of NO, weigh -2016 grm. Vapour density = 2°588. 
Here weak solution of caustic potash (sp. gr. = 1:0045) was employed, and 
considered to have the same tension as pure water. 
The vapour density of NO,, given in Recnavutt’s “Cours Elémentaire de 
Chemie,” is 1-72,—a number which is a little less than the first number on the 
following list :— 
C. Vapour Densities. 
Obs. latyy-o —o.. ‘ : : ¢ 1-783 
PRD uPA?) OMT intl be asl ooy heads eat 
eee TIns Ty ea ae Ne  . aeee emGNs 
ine aa ee ale Tk) ae 
The formula NO, requires = x 0691 = 1°5893. 
The formula N,O, requires ee x 0691 = 3°1786. 
It will be observed that even at 97-5° C. the vapour density of peroxide of — 
nitrogen is rather too high for the first formula, whilst at 24°5° and downwards, 
the vapour density is much nearer that required by the second formula than that 
required by the first. 
If it be allowed that there exist two peroxides of nitrogen, NO, and N,0O,, 
and that these bodies pass readily into one another, the above resuls find a 
simple and natural explanation. At 100° C., the gas consists chiefly of NO,; at 
ordinary temperatures, chiefly of N,O,; whilst at intermediate temperatures, — 
intermediate quantities of NO, and N,O, are present. 
If change of composition be denied, the peroxide of nitrogen must have an 
expansion-coefficient widely differing from that of other gases. To admit such 
an exception to one of the best physical generalisations seems unphil¢:oraa 
and contrary to the usual proceeding of science. : 
In general, gases when exposed to pressures not exceeding the usual baro- 
metric pressure, and when no incipient liquefaction is taking place, have an 

