i8o PROGRESS OF SCIENCE. 



in weights of 16 or multiples of 16 hence the name of the 

 j aw and the law applies to all elements, each having its 

 particular weight ; carbon will combine in weights of 12 or 

 multiples of 12 ; nitrogen will combine in weights of 14 or 

 multiples of 14 (28, 42, 56, etc.) ; sodium in weights of 23 

 (then 46, 69, etc.) ; phosphorus in weights of 31 (62, 93, etc.) ; 

 iron in weights of 28 (56, 84, etc.) ; arsenic, 75 (then i$o, 225, 

 300, etc.) ; mercury, 100 (then 200, 300, 400, etc.). The 

 third law on which chemistry rests is the LAW OF ATOMIC 

 or equivalent PROPORTIONS, which is that "each element in 

 combining with^ or displacing other elements, does so in a 

 fixed proportion" which may be stated numerically the 

 weights just mentioned being equivalent to the parts : if a 

 slip of copper, for instance, be introduced into a solution of 

 mercuric chloride, proportions of the two metals will change 

 places, since chlorine has a stronger affinity for copper than 

 mercury : cupric chloride is formed and mercury deposited 

 for every 31*7 parts by weight of copper dissolved, 100 of 

 mercury are separated. The atomic theory embraces the 

 question of atomic weight, atomic volume, atom-fixing 

 power, and so forth. The laws involved in the theory, if not 

 all discovered, were all first brought into the light of in- 

 tellectual day, by Dalton. Whether the doctrine of atoms 

 be absolutely true, as it seems to be now admitted, or a 

 working hypothesis only, as some are still contending, 

 Dalton's atomic theory (as distinguished from the doctrine of 

 atoms) is admitted by every philosophical chemist, and is 

 rendering new services to chemistry every day. Dalton 

 meant by an atom <f the smallest possible quantity of any 

 element which can combine with other substances." Dalton 

 fixed the atomic weight of hydrogen, but the atomic weight 

 of elements has not in all cases been determined, nor in some 

 cases determined uniformly some chemists (Berzelius, 

 Olding, Gerhardt, Watts) differing from one another slightly 

 in their determinations. The atomic system, it should be 

 remembered, has given rise to a system of symbols which 

 has become the universal language of chemistry a very 

 great advantage to science. Dalton investigated also the 

 dilatation of gases, and numerous chemical phenomena. 



