44 THE ERRORS OF PRECISION. 



So long as it is impossible to obtain absolutely pure 

 crystals of borax, having exactly all the theoretical water of 

 crystallization and no more, it is absurd to weigh more 

 accurately than to the milligramme. 



The Man and the Balance. 



In concluding this most instructive episode from the 

 history of atomic weight determinations, I trust the reader 

 hereafter, when studying some new atomic weight determi- 

 nation and noticing how the accuracy of the balance and 

 weights used is extolled, will not conclude that this guaran- 

 tees accuracy in the final results. 



If the man behind the gun tells on the result in battle, 

 the chemist before the balance tells on the resulting atomic 

 weight. 



What has been said may suffice on the subject of weigh- 

 ing. A few words are still required on the calculation of 

 the analytical ratio, especially as to the number of decimals 

 that ought to be retained. 



Of course, according to the novice it is only a question 

 of physical endurance and space which limits the number 

 of decimals in the quotient calculated from the two observed 

 weights of product and substance. 



Official Rule. 



Sometimes it is Krule officially given, independent of the 

 case in hand. I am afraid this is quite often the only limit 

 observed. 



I vividly recollect how, more than thirty years ago, I 

 noticed the specific gravity of limestone specimens marked 

 to seven decimal places each, without fail. 



This was in a great scientific military establishment 

 supported by Uncle Sam. 



Knowing how sensitive military scientists are, I ran my 

 eye over a large number of the samples of buiding stones, 

 con-esponding samples of which were known to be in my 

 own hand for investigation, before I dared ask for " more 

 light." 



