140 



An inspection of all the above results shows that the rarefaction 

 of air, from atmospheric pressure downwards, produces a uniformly 

 diminishing illuminating power until the pressure is reduced to about 

 14 inches of mercury, below which the diminution of light proceeds 

 at a less rapid rate. The above determinations give approximately 

 5'1 per cent, as the mean reduction of light for each diminution of 

 1 inch of mercurial pressure down to 14 inches. The following 

 Table exhibits the actually observed light, compared with that 

 calculated from this constant. 



1st Series. 



I am now extending this inquiry to pressures exceeding that of 

 the atmosphere, and hope soon to lay before the Society the detailed 

 results of the whole series, together with some observations on the 

 causes of this variation of luminosity. 



