438 MODERN CHEMISTRY. 



an amount of alloy much less than a thousandth part their 

 own weight. And there are cases where a proportion of cal- 

 careous matter, much smaller still, suffices to alter the sensible 

 properties of the substances through which it is diffused. 



So numerous, however, are the instances of this nature in 

 Chemistry, that the citation of a few rather impairs than 

 enlarges the conception of the great principle they involve. 

 They form, in truth, part of that great attainment of modern 

 science, the discovery of conditions and laws of molecular 

 change in the interior of bodies of those subtle inter- 

 penetrations of matter under the influence of light, heat, 

 electricity, or chemical force, and often independently of the 

 outer forms or densities of the bodies acted upon the study 

 of which brings us among the more occult relations of the 

 material world, and promises access to physical truths yet 

 higher than any hitherto attained. 



In estimating the influence of these small quantities in 

 composition, some chemists, and particularly Liebig and 

 Dumas, have pursued a method, partially adopted before but 

 in a manner far below the perfection of present use. This 

 consists in calculating, through the proportion of parts, the 

 absolute or approximate quantities of matters thus minutely 

 diffused; expressing them in weights or volumes; and 

 thence deriving results inaccessible by other modes of re- 

 search. The positive amount of carbon present in the at- 

 mosphere, for example, is a question of much interest to the 

 theory of vegetation, and to other phenomena of the earth's 

 surface. This question is solved, first, by estimating (which 

 can be done exactly) the total weight of the atmosphere sur- 

 rounding the globe; next, by taking the fractional proportion 

 which carbonic acid forms of this amount ; and, finally, by 

 deducting the further proportion which oxygen bears in the 

 composition of the carbonic acid ; giving as a gross result 

 3,085 billions of pounds of the element of carbon existing in 



