422 On the Theory of Mixed Gases, ' 



before it can effect an entrance, according to 

 Mr. Dalton's own experiments. 



The preceding arguments are derived from 

 self-evident principles -, and they discover the 

 reason, why Mr. Pictet and others have proved 

 unable to do away the objections to which 

 the weight of the air exposed their favourite 

 notion. In reality the objections appear to be 

 unanswerable ; because the notion has been 

 demonstrated to be repugnant to the mecha- 

 nical philosophy. Mr. Dalton lately invented 

 an ingenious correction of M. de Luc*s hy- 

 pothesis, according to which, vapour, in pas- 

 sing through a stratum of air, meets with a re- 

 sistance from the inertia of the gaseous par- 

 ticles, similar to the opposition a current ex- 

 periences in making its way through a bed of 

 pebbles. The inference that follows this 

 familiar illustration is obvious ; namely, that 

 the particles of air remain immoveable like the 

 pebbles, the one doing nothing, but obstruct 

 the progress of the steam., just as the other 

 diminishes the velocity of the water. But 

 though the merit of my friend's, corrected hy- 

 pothesis depends upon the establishment of this 

 new idea, he seems desirous to abandon it at 

 present, and maintains that vapour of low tem- 

 peratures does enlarge the pores of air ; he must 

 therefore be contented to rank with AL Pictet, 



