674 The Outline of Science 



within the ordinary limits of experiment, it is identical with the 

 amount of energy which would have been produced in the form of 

 heat, if the food supplied to him had been burnt. The principle 

 of the conservation of energy thus holds good for a dog or a man 

 as well as for a steam-engine or a dynamo; no special "vital 

 energy" is being mysteriously introduced. 



It is the same with the chemical composition of living organ- 

 isms. There are no elements in our bodies which do not exist in 

 lifeless matter; and, indeed, the chemical constituents of living 

 matter are all to be reckoned among the commoner elements. The 

 bulk of living substance is composed of four ubiquitous elements- 

 carbon, hydrogen, nitrogen, and oxygen ; besides these, iron, phos- 

 phorus, sulphur, sodium, potassium, calcium, chlorine, and prob- 

 ably iodine seem to be universally present. 



It was at one time supposed that a distinction between living 

 and non-living matter could be drawn in respect of their powers 

 of chemical construction. Many chemical substances, such as 

 starch, sugar, albumen, urea, and so forth, are only to be found 

 normally as the product of living organisms ; and it was supposed 

 that the "vital force" concerned itself with the manufacture of 

 such special compounds. This again, however, has proved not to 

 be founded in fact. In the middle of last century, Wohler suc- 

 ceeded in synthesising urea out of lifeless matter in a test-tube. 

 Since that day more and more organic substances have been 

 artificially made. 



The most complicated chemical substances which have been 

 isolated from living matter are the proteins ( such as the albumen 

 of egg white), consisting, as they do, of hundreds, often thou- 

 sands, of atoms; these are combined first of all into comparatively 

 simple compounds, called amino-acids, and the amino-acids in 

 their turn are linked together in definite ways to form the hund- 

 reds and thousands of different proteins that we know. 



Emil Fischer has succeeded in artificially linking together a 

 "limber of amino-aci^s to form a very complicated synthetic 



