VON HUMBOLDT 187 



years later, when the same result was obtained in an equally 

 well supported manner, namely from the kinetic theory of 

 gases of Robet Mayer and Clausius. 'Avogadro's law' of 

 equal numbers of molecules in equal volumes of all gases (at 

 the same pressure and temperature) is one of thd best ascer- 

 tained facts of science. It is the foundation of all deter- 

 minations of molecular weight and hence of atomic weight. 



Gay-Lussac was also the first to investigate quantitatively 

 the observation made shortly before by Dalton, that gases 

 when suddenly compressed become warm, and when ex- 

 panded are cooled, the latter only happening when expan- 

 sion takes place against external pressure, and not when they 

 flow into a vacuum.^ 



Alexander von Humboldt, son of Major Georg von Hum- 

 boldt, who served in the Seven Years War, received together 

 with his brother Wilhelm, who was two years older, a careful 

 education at the family seat at Tegel, near Berlin. Neither of 

 the brothers went to school until they entered the university. 

 After many years of varied study, Alexander set out on ex- 

 tensive travels into the interior of North and South America, 

 and to Asia, concerning which he published a very large 

 work containing many geographical, meteorological, zoolo- 

 gical, mechanical, geological, historical, and other results. 

 From the year 1827, yielding to the earnest desire of the 

 King of Prussia, he lived permanently in Berlin. He first 

 gave lectures on physical geography to a very large circle of 

 hearers at the university, and then turned his attention to 

 writing his second great work, the Kosmos. The leading 

 idea of this he himself states in a letter as follows: 'I have 

 been seized with the mad idea of representing in a single 



1 The investigation in which Gay-Lussac proved the latter fact in the 

 year 1807, later became of particular importance since Robert Mayer 

 was able to use it as an unexceptionable basis for calculating the mechani- 

 cal equivalent of heat; it is reprinted in the appendix to Mach's Wdrme- 

 lehre. 



