634 HISTORY OF SCIENCE. 



ment of the atoms constituting the molecule. Such substances are 

 said to be isomeric. We have just seen the theoretical possibility of 

 two isomeric hydro-carbons C 4 H IO , and of three C 5 H I2 's ; and so we 

 might deduce the possibility of four different C 6 H I4 's, six C 7 H l6 's, and 

 so on. Now, it is a fact that in several cases all the isomeric com- 

 pounds foreseen by the theory have been recognized as actually existing 

 substances. In other cases only some of the isomerides have yet been 

 prepared. The facts of isomerism thus lend a very efficient support to 

 the atomic theory. For example, there are two hydro-carbons known, 

 each of which contains 8277 per cent, of carbon and 17*23 per cent, 

 of hydrogen, and in the state of vapour is just thirty times as heavy 

 as hydrogen gas under the same circumstances of temperature and 

 pressure. These facts show that the molecular formula for both is 

 C 4 H I0 . They are, however, different substances, being prepared in 

 different ways, and possessing distinctly different properties in certain 

 respects, although in general chemical character they resemble one 

 another. For instance, CH 3 . CH . CH 2 . CH 3 is a liquid which boils 

 at 34 F., while CH(CH 3 ) 3 boils at 5 F. Of isomeric C 5 H I2 's three 

 have been prepared. Of the next member of the series, C 6 H I4 , only 

 three modifications are known; it is presumed, therefore, that one 

 remains yet to be discovered. 



DALTON (from the Statue by Chantrey). 



