SECT. m. SEQUENCE OF ATOMIC NUMBERS. 105 



meric series of a gas, a liquid and a solid, consisting of 

 carbon and hydrogen. The gas contains 86 parts in 

 100 of carbon, and forms the most luminous part of 

 coal gas. 



M. Dumas has proved it to be a general law, that 

 when three isomeric bodies are arranged in the 

 sequence of their chemical properties, there will also 

 be a sequence in their respective atomic numbers, and 

 that whenever this symmetry of chemical properties 

 and atomic weights obtains, any one of these substances 

 may be substituted for the other without changing the 

 chemical character of the formula. 



Sulphur, selenium, and tellurium, form an isomeric 

 group ; that is, they form a sequence, with analogous 

 qualities, for sulphur is the most volatile ; selenium, a 

 simple substance found in iron pyrites in Sweden, is 

 less volatile ; and tellurium is the least volatile and with 

 regard to their atomic sequence, the atomic weight of 

 sulphur is 16, that of tellurium is 64, and half the 

 sum of these numbers is 40, the atomic weight of sele- 

 nium, the mean term. Hence selenium might be put in 

 any compound for the sulphur, and the tellurium for 

 the selenium, without changing the chemical character 

 of its formula. 



The metallic group of calcium, strontium, and barium, 

 are endowed with analogous properties, perfect harmony 

 in their chemical qualities, and in the numbers express- 

 ing their atomic weights. That of calcium is 20, that 

 of barium is 68, and the half sum is 44, the atomic 

 weight of strontium. So calcium might be put for 

 strontium, and strontium for barium, in any compound 

 without altering the character of its formula. Professors 

 Johnson and Allen have shown that the new metalloids 

 caesium and rubidium form an isomeric triad with 

 potassium, for the atomic weight of caesium is 133, that 

 of rubidium 86, and that of potassium 39. 



Transmutations of one isomeric substance for an- 



