160 



SCIENCE 



[N. S. Vol. XXXIII. No. 840 



law of the conservation of energy. The 

 marvelous effect of this in enabling one to 

 correlate and remember apparently unre- 

 lated facts was brought out in a most 

 striking way in Kundt's lectures on 

 physics. 



I wish to call your attention to-night to 

 what I believe to be another universal law, 

 a qualitative one and not a quantitative 

 one. The chemists call it the theorem of 

 Le Chatelier. The physicists call it the 

 theorem of De Maupertuis or the principle 

 of least action. By the biologists it is 

 known as the law of the survival of the 

 fittest, while the business man speaks of 

 the law of supply and demand. The 

 broadest definition of it is that a system 

 tends to change so as to minimize an ex- 

 ternal disturbance. 



In chemistry proper and in physics we 

 study chiefly the effects of temperature, 

 pressure, concentrations, electricity and 

 light; but in the natural-history sciences 

 we must also take account of moisture, 

 food and fertilizers, secretions, climate, etc. 



If we heat a liquid we convert a portion 

 of the liquid into vapor, an operation 

 which absorbs heat. If we heat a satu- 

 rated solution, the solubility increases if 

 the solid dissolves with an absorption of 

 heat. If we increase the pressure on a 

 dissociating compound or if we increase the 

 concentration of the dissociation products, 

 we get a decrease of dissociation which in- 

 volves a decrease in pressure and a de- 

 crease in the amount of the dissociation 

 product. If we pass an electric current 

 through a solution, we tend to get a coun- 

 ter-electromotive force which cuts down 

 the electrical stress. If we have suspended 

 particles in a liquid, a difference of poten- 

 tial causes them to move in the direction 

 which reduces the electrical stress. Since 

 all substances absorb light of some wave- 

 length to a greater or less extent, all 



substances are light-sensitive to some rays 

 and tend to change in such a way as to 

 eliminate the strain caused by the light. 

 Whether any measurable change takes 

 place depends on other conditions. With 

 some silver salts or with Eder's solution of 

 mercuric oxalate, we get visible decomposi- 

 tion. With chromium salts we get no 

 measurable change unless some reducing 

 agent is present. With some substances 

 we get fluorescence or phosphorescence, but 

 ordinarily without apparent change. With 

 a copper sulphate solution there is appar- 

 ently no effect due to light. Yet all these 

 solutions are really light-sensitive and they 

 all tend to change in the same way, namely, 

 to eliminate the substance which absorbs 

 the light. 



In the business world continuous over- 

 production of any commodity necessitates 

 a fall in the price of the article. Also, a 

 sufficiently wide-spread and prolonged de- 

 crease in the demand for meat will bring 

 down the price, at least temporarily. This 

 may not help the consumer, because he 

 may have to pay more for fish, eggs and 

 vegetables ; but that is not our problem. 



If one should ship enough copper abroad 

 as export copper and should bring it back 

 as ballast, one would create a state of 

 strain which would eventually play havoc 

 with the price of copper. It is said that 

 this experiment was tried when the price 

 of copper was over twenty cents; but the 

 details have never been published. Over- 

 production of silver destroyed the ratio of 

 sixteen to one, while over-production of 

 gold is now said to be one of the factors 

 in the high cost of living. 



In physiology and medicine we find 

 many illustrations of our law. "When 

 irritating substances get into the eye, a 

 fiow of tears occurs to wash them away; 

 from the nose and respiratory passages, 

 they are ejected by sneezing or by cough ; 



