CH. xxxvn. D ALTON'S ATOMIC THEORY. 405 



died 1850), adopted it at once, and added another discovery 

 in favour of it in 1809 namely, that when substances are 

 reduced to gas, and the gas is collected, it is found that the 

 different elements combine in equal or multiple volumes. 



You will understand this by turning back to the com- 

 pounds of nitrogen and oxygen (p. 401), where you will see 

 that there was always either i, 2, 3, 4, or 5 volumes of 

 oxygen collected for one of nitrogen, and never a part of 

 a volume. This was really a different fact from the one 

 Dalton pointed out, that the elements combine in definite 

 weights, and it was necessary to complete the law of mul- 

 tiple proportions. In 1811 a still further step in advance 

 was made by the Italian chemist Avogadro, who showed 

 that equal volumes of gases or vapours under the same 

 conditions of pressure and temperature contain the same 

 number of molecules ; and Ampere brought forward the 

 same doctrine in 1814. These discoveries were followed up 

 by Gerhardt, Berzelius, Sainte-Claire, Deville, and others. 



We said at p. 398 that it is possible those bodies which 

 we now call elements may not after all prove each one to be 

 a simple substance, and in the last few years many new 

 theories have been formed on this subject. Mr. Lockyer 

 has been led by his study of the spectrum of the sun to 

 the opinion that many of the substances which behave as 

 elements in our globe are dissociated or split up in the 

 fierce solar heat ; while Mr. Crookes, from chemical experi- 

 ments, and from the double forms of crystallisation of 

 certain elements, concludes that there are some far more 

 unstable than others, and he proposes to call these meta- 

 elements. There are many difficulties as yet in proving 

 either of these hypotheses, but it is well to remember that 

 chemists and physicists are both now inclined to question 

 the simple nature of many so-called elements. 



