146 



Lord Rayleigk. 



[Mar. 23, 



Although the subject is not yet ripe for discussion, I cannot omit 

 to notice here that nitrogen prepared from ammonia, and expected to- 

 be pure, turned out to be decidedly lighter than the above. When 

 the oxygen of air is burned by excess of ammonia, the deficiency is 

 about 1/1 000th part.* When oxygen is substituted for air, so that 

 all (instead of about one-seventh part) of the nitrogen is derived from 

 ammonia, the deficiency of weight may amount to ^ per cent. It 

 seems certain that the abnormal lightness cannot be explained by 

 contamination with hydrogen, or with ammonia, or with water, and 

 everything suggests that the explanation is to be sought in a dis- 

 sociated state of the nitrogen itself. Until the questions arising out 

 of these observations are thoroughly cleared up, the above number 

 for nitrogen must be received with a certain reserve. But it has not 

 been thought necessary, on this account, to delay the presentation of 

 the present paper, more especially as the method employed in prepar- 

 ing the nitrogen for which the results are recorded is that used by 

 previous experimenters. 



Reduction to Standard Pressure. 



The pressure to which the numbers so far given relate is that due 

 to 762-511 mm. of mercury at a temperature of 14'85°,t and under the 

 gravity operative in my laboratory in latitude 51° 47'. In order to 

 compare the results with those of other experimenters, it will be con- 

 venient to reduce them not only to 760 mm. of mercury pressure at 0°, 

 but also to the value of gravity at Paris. The corrective factor for 

 length is 760/762'511. In order to correct for temperature, we will 

 employ the f ormulaj 1 + 0-0001818 1 + 0-00000000017 t 2 for the volume 

 of mercury at t°. The factor of correction for temperature is thus 

 1*002700. For gravity we may employ the formula — 



g = 980-6056-2-5028 cos 2\, 



X being the latitude. Thus, for my laboratory — 



g = 981-193, 



and for Paris — 



g = 980-939, 



the difference of elevation being negligible. The factor of correction 

 is thus 0-99974. 



The product of the three factors, corrective for length, for tempera- 

 ture, and for gravity, is accordingly 0*99914. Thus multiplied, the 

 numbers are as follows : — 



* ' Nature,' vol. 46, p. 512. 



f The thermometer employed with the manometer read 0-15° too high. 

 X Everett, p. 142. 



