INTRODUCTORY PROBLEMS 5 



crystal and the growing caterpillar, the piece of potassium and 

 the whirligig beetle.] 



(6) From the Physicist's Point of View. The living creature is 

 like some wonderful kind of engine or machine, but it is more 

 efficient. Like an engine it transforms matter and energy, but 

 it does so more economically. It is a self-stoking, self-repairing, 

 self -preservative, self-adj us ting, self -increasing, self -reproducing 

 engine ! It has a remarkable power of accumulating and storing 

 energy, of taking rests, of " acting of its own accord," of profiting 

 by experience. In comparing a living creature with a machine, 

 which is a very useful analogy to work out with pupils advanced 

 enough to understand it, it should always be remembered that 

 a machine is hardly a fair sample of the inorganic world, since it 

 is the embodiment of a human thought. 



(c) From the Chemist's Point of View. The component elements 

 of living creatures are just the common elements found in their 

 surroundings, but the " make-up " of the compounds which form 

 the physical basis of life is very intricate. The chemical elements 

 which enter into the composition of organisms are : carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potas- 

 sium, sodium, calcium, magnesium, and iron ; and there may 

 be others. Of these elements the first five enter into the com- 

 position of the most characteristic of organic compounds which 

 are invariably present in living creatures, namely, the proteids, 

 such as the albumin of eggs, the haemoglobin of our blood, or 

 the gluten of wheat. It is probable that living matter is a mixture 

 of proteids, owing its virtue to their co-operative interaction, 

 just as the secret of a firm's success may depend not on any 

 one partner by himself, but on the combination of talents. In 

 the living body we can trace a number of chemical processes 

 which follow one another in a sort of routine. We can trace the 

 raw material of food being digested and incorporated into the 

 living body ; we can detect the breaking-down of complex sub- 

 stances and the getting rid of waste ; but we cannot redescribe 

 the unified life of the creature in terms of chemical formulae. 

 It seems certain, however, and this is perhaps the most significant 

 feature from the chemist's point of view, that there is a twofold 



