630 



HISTORY OF SCIENCE. 



take part in reactions ; and when the thermic values of these have 

 afforded the requisite data, one principle will enable us always to 

 define the results of any given reaction, the principle of maximum 

 work. The work is measured by the quantity of heat developed in a 

 chemical action. In the estimation of this quantity, allowance must 

 be made for merely physical changes, and the substances must be re- 

 ferred to one state. M. Berthelot, in attempting to construct a theory 

 of therm o-chemistry in 1864, found so many voids and uncertainties 

 in the data, that he undertook a prolonged and laborious series of ex- 

 periments extending over sixteen years, the results being announced 

 from time to time, and finally embodied in two large volumes recently 

 published by him. 



Now, as an illustration of the manner in which the atomic theory is 

 applied in modern chemistry, we may take the doctrine of series in 

 carbon compounds. A chemist named SCHIEL (1842) appears to have 

 been the first to call attention to the fact that a certain class of com- 

 pounds (alcohols) form a regularly progressive series, in composition 

 and in properties. Gerhardt and Laurent made this the principle of a 

 systematic arrangement of the carbon compounds. As an example, a 

 series may be mentioned, the very numerous compounds of which 

 consist of nothing but carbon and hydrogen. Some of these are gases, 

 others are liquids or solids at ordinary temperatures. Experiment 

 shows the relative quantities of the two elements contained in each 

 compound, and the density of the substance, that is, as to how many 

 times the gas, or the liquid or solid raised by heat to the state of gas, 

 is heavier than pure hydrogen gas at the same temperature and pres- 

 sure. From the former the ratio between the number of carbon atoms 

 and the number of hydrogen can be inferred, the carbon being known 

 from other considerations to be twelve times as heavy as the hydrogen 

 atom. The density (or specific gravity) determines (by Avogadro's law) 

 the actual numbers by which the ratio is to be expressed, for it tells 

 how much heavier is the molecule of the gas or vapour than the mole- 

 cule of hydrogen (page 613). These relations will be seen in the 

 following table of a set of hydro-carbons. 



