DALTON 183 



and Mariotte, and the vapour pressure, which only depends 

 upon the temperature, do not interfere with one another; they 

 simply add together. But if the vapour is not saturated, that 

 is, if no excess of liquid is present, it behaves like a gas and 

 follows Boyle's law. 



In this connection, he also investigated thoroughly the 

 change of volume of gases on heating under constant pres- 

 sure. He saw that the data then available could not be of 

 much value, since they came from unsatisfactory experi- 

 ments; the gases had been enclosed over water, and hence gas 

 and vapour had been measured together. He investigated 

 gases dried as thoroughly as possible; air, hydrogen, oxygen, 

 carbonic acid, and nitric oxide, and he showed that they all 

 expand by an equal amount for each degree, or from the ice 

 point to the boiling point by 100/265, of the volume at the 

 ice point. This is the law published in the same year (1802), 

 but a little earlier, by Gay-Lussac in Paris, and to-day named 

 almost exclusively after the latter.^ The co-efficient of 

 expansion of gases as later determined with increasing ac- 

 curacy, 1/273, ^s a little smaller than Dalton's value (and 

 also that of Gay-Lussac). This co-efficient of expansion of 

 gases became of especial importance after the insight fur- 

 nished by the kinetic theory of gases founded by Clausius, 

 since it gives us the absolute zero of temperature, - 273°C, 

 at which the volume maintained by the motion of the gas 

 molecules, and hence the kinetic energy of these molecules - 

 the true measure of the temperature - becomes zero. 



Dalton also used his vapour pressure table for measuring 

 the quantity of moisture in the air, in exactly the same way as 

 is done with the present day dewpoint hygrometer, of which 

 he is therefore the inventor, although it later assumed a more 

 convenient form. 



1 In older publications, for example Carnot's important work on heat 

 engines (1824) it is called the law of Gay-Lussac and Dalton, which is as 

 well justified as the usual description of the pressure law as of Boyle and 

 Mariotte. 



