370 



pressed atmospheres upon the light of combustion. Great diffi- 

 culties were experienced in this branch of the inquiry, as gas could 

 not be used, and recourse must therefore be had to other combus- 

 tibles, which, as already pointed out, are liable to certain irregularities. 

 Owing to these and other difficulties, satisfactory determinations 

 could only be made between one and two atmospheres. In these 

 determinations, the lamp which replaced the experimental gas flame 

 was supplied with amylic alcohol a liquid which, whilst affording 

 an appreciable amount of light under one-atmosphere pressure, was 

 found to burn under two atmospheres without smoke, although at 

 a somewhat higher pressure it began to evolve unconsumed carbon. 

 The results obtained approximate closely to those indicated by the 

 above law, deduced from the corresponding determinations in rare- 

 fied atmospheres, as will be seen from the following table, in which, 

 the mean of eleven observations is given under each experiment ; the 

 column headed " calculated" containing the numbers deduced from 

 the rate of variation of luminosity in rarefied air : 



Further determinations, in which the illuminating power at three- 

 and four-atmospheres pressure was compared, yielded results differ- 

 ing widely from this law, and indicating a much more rapid increase 

 of light ; but as the liability to errors increases greatly at these 

 higher pressures, little confidence is placed in the numbers. The 

 lamp was fed with a mixture of five parts of vinic alcohol and one 

 part of amylic alcohol ; it had no appreciable illuminating effect 

 under ordinary atmospheric pressure : 



