xv THE MOLECULAR THEORY 327 



671). During the next hundred years many attempts were 

 made to measure this expansion ; but the results were very 

 irregular, mainly because the air, and the vessels in which it 

 was contained, were not sufficiently dried. 



Still greater irregularities were observed when Priestley, in 

 1777 ("Experiments on Air," III. 345-348), measured the ex- 

 pansion by heat of different gases and found that 



" The different kinds of air are expanded by the addition 

 of ten degrees of heat according to Fahrenheit's thermo- 

 meter, in the following proportion. 



Common air I '32 



Inflammable air 2 '05 



Nitrous air 2'O2 



Fixed air 2*20 



Marine acid air I '33 



Dephlogisticated air 2*21 



Phlogisticated air 1-65 



Vitriolic acid air 2*37 



Fluor acid air 2 '83 



Alkaline air 475 



(Experiments and Observations, V. 359). 



The fact "that oxygen, azote, hydrogen and carbonic 

 acid, and atmospheric air expand equally between o and 

 80 " was discovered by Charles about the year 1787, but 

 his results were never published (Gay-Lussac, Ann. de 

 Chimie, 1802, 43. 157). The more detailed experiments 

 " On the dilatation of gases and vapours," which were 

 described by Gay-Lussac in 1802 (loc. at. 137-175), were 

 made with the apparatus shown in Fig. 47, or with a modified 

 apparatus in which the iron tap was replaced by a mercury 

 valve. The expansion between the freezing-point and the 

 boiling-point of water was measured by heating the gas to the 

 temperature of boiling water, opening the tap to allow the 

 excess of gas to escape, then cooling in ice and weighing the 

 water that was drawn in when the tap was again opened under 

 water. Six experiments with common air showed that 



