THE ATOMIC VIEW OF NATURE. 



425 



not surer to us now than is the atomic or molecular 

 theory in chemistry and physics so far, at all events, as 

 its assertion of heterogeneousness in the minute structure 

 of matter, apparently homogeneous to our senses, and to 

 our most delicate direct instrumental tests." 1 



This side of the atomic view of matter has been de- 

 veloped by the study of the properties of bodies in the 

 gaseous state, and, in its modern form, goes back to the 

 experiments of Gay-Lussac, which were almost simul- 

 taneous with those of Dalton. 2 It is interesting to note 

 how little the latter recognised the importance of these 

 researches, when he rejected the so-called law of volumes, 

 according to which gases, under the same pressure, and 

 at equal temperatures, enter into, or separate out of, 

 chemical combination in definite and very simple pro- 

 portions of their volume. As, according to the law of 

 definite proportions, bodies (including gases) combine only 



1 Lord Kelvin on ' ' Capillary At- 

 traction," 1886. See ' Popular Lec- 

 tures and Addresses,' vol. i. p. 4. 



2 The first results referring to 

 the combining volumes of oxygen 

 and hydrogen gas in forming water 

 were given by Gay - Lussac and 

 Humboldt in a joint memoir. Their 

 experiments were carried on in 

 1805. Gay-Lussac continued the ex- 

 periments alone, extended them to 

 gaseous compounds, and published 

 his results in 1809 in the second 

 volume of the ' Memoires d'Arcueil.' 

 This was one year after the publi- 

 cation of Dalton's ' New System 

 of Chemical Philosophy,' and two 

 years after Thomas Thomson had 

 published a sketch of the atomic 

 theory in his text-book on Chem- 

 istry. The law of equal expansion 

 of all gases with temperature was 

 published by Dalton in 1801 ; the 



law of pressures that the volume 

 of a gas, at the same temperature, 

 is inversely as the pressure was 

 published by Boyle in 1662. It 

 goes on the Continent under the 

 name of Mariotte, who first made 

 it generally known about twelve 

 years later (see on this the fourth 

 appendix to the 2ud edition of 

 Tait's ' Properties of Matter,' 1890). 

 The law of temperatures was pub- 

 lished in 1802 by Gay-Lussac in 

 the ' Annales de Chimie et de Phys- 

 ique" (vol. xliii. p. 137), where he 

 remarks that Charles, Professor of 

 Physics at the " Conservatoire," had 

 fifteen years earlier noted the prop- 

 erty indicated by this law. Both 

 these so-called laws of gases are 

 only accurate within certain [not 

 very wide limits of temperature and 

 pressure. 



28. 



The kinetic 



