ECONOMIC QUALITY CONTROL OF PRODUCT 369 



If SO, what will be the cause? These questions we cannot answer. 

 Some of us are to fall at one time from one cause, others at another 

 time from another cause. In this fight for life we see then the element 

 of uncertainty and the interplay of numerous unknown or chance 

 causes. 



However, when we study the effect of these chance causes in produc- 

 ing deaths in large groups of individuals, we find some indication of a 

 controlled condition. We find that this hidden host of causes produce 

 deaths at an average rate which does not differ much over long periods 

 of time. From such observations we are led to believe that, as we 

 approach the condition of homogeneity of population and surroundings, 

 we approach what is customarily termed a "Law of mortality" such as 

 indicated schematically in Fig. 1. In other words, we believe that in 

 the limiting case of homogeneity the causes of death function so as to 

 make the probability, let us call it dy, of dying within given age limits, 

 such as forty-five to fifty, constant: That is, we believe these causes are 

 controlled. In other words, we assume the existence of a kind of 

 statistical equilibrium among the effects of such an unknown system 

 of chance causes expressable in the assumption that the probability of 

 dying within a given age limit, under the assumed conditions, is 

 an objective and constant reality. 



B. Molecular Motion 



Just about a century ago, in 1827 to be exact, an English botanist. 

 Brown, saw something through his microscope that caught his interest. 

 It was motion going on among the suspended particles almost as though 

 they were alive. In a way it resembled the dance of dust particles in 

 sunlight, so familiar to us, but this dance differed from that of the dust 

 particles in important respects — for example, adjacent particles seen 

 under the microscope did not necessarily move in even approximately 

 the same direction, as do adjacent dust particles suspended in the air. 



Watch such motion for several minutes. So long as the temperature 

 remains constant, there is no change. Watch it for hours, the motion 

 remains characteristically the same. Watch it for days, we see no 

 difference. Even particles suspended in liquids enclosed in quartz 

 crystals for thousands of years show exactly the same kind of motion. 

 Therefore, to the best of our knowledge there is remarkable permanence 

 to this motion. Its characteristics remain constant. Here we cer- 

 tainly find a remarkable degree of constancy exhibited by a chance 

 system of causes. 



Suppose we follow the motion of one particle to get a better picture 

 of this constancy. This has been done for us by several investigators, 



