THE PHYSICAL BASIS OF LIFE 177 



of quite dissimilar-looking substances. A man 

 brings guano from some South American island 

 and puts it beside the molecules of a rose, and 

 the molecules of the rose take atoms from the 

 guano and weave them into the warp and woof 

 of petals, and stamens, and roots. A man burns 

 up several pounds of his tissues by violent 

 exercise, and he puts some cheese or some 

 potatoes into contact with certain molecules in 

 his stomach — the molecules of the so-called diges- 

 tive glands — and the stomach molecules pull the 

 molecules of the cheese or potatoes to pieces 

 and rebuild them into protoplasm — into com- 

 pounds containing carbon, hydrogen, oxygen, and 

 nitrogen, in certain definite proportions. 



Let us look for a moment at the atoms com- 

 prising the molecule of protoplasm. Oxygen, 

 nitrogen, and hydrogen are all gases at ordinary 

 temperatures. Carbon, again, is a solid at ordi- 

 nary temperatures, and only at very extraordinary 

 temperatures, a gas : it occurs in such different 

 forms as graphite, charcoal, diamond, and in com- 

 bination with oxygen it forms the colourless gas, 

 carbonic acid gas. Carbon also has an extraordinary 

 faculty of linking a number of atoms together and 

 forming large molecules. Thus the carbon com- 

 pound, stearine;, has a molecule consisting of no 

 fewer than one jhundred and eighty-three atoms. 

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