IO ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



From these considerations it was clear that the reason for the 

 incompatibility of the results on sodium was probably to be sought 

 chiefly in the method of analysis rather than in the impurity of the 

 materials. The long-continued search for the causes of discrepancy 

 led finally to a satisfactory explanation of the whole anomaly. In 

 order to make this clear, the details of our own investigation must 

 be recounted. 



BALANCE AND WEIGHING. 



The Troemner balance which has served in many similar researches 

 was used during most of the present work.* Successive weighings on 

 it of the same object rarely differed as much as 0.03 milligram. For 

 a few of the conclusive final experiments a yet more sensitive and 

 perfect balance, especially made for this kind of work by the same 

 maker, was used. 



The Sartorius weights were standardized from time to time by the 

 method devised by one of us.f It was found as usual that the larger 

 gold-plated and platinum-plated brass weights very slightly changed 

 from time to time, although the smaller platinum weights did not. 

 All weighings were made by substitution. Furthermore, the major 

 portion of the substituted tare consisted of a counterpoise exactly sim- 

 ilar to the crucible, tube, or boat which was being weighed. Thus the 

 weights required were never large in amount and errors due to changes 

 in meteorological conditions were avoided. The method of recording 

 and checking the weights is given in another place .$ 



While these obvious particulars are given here for the sake of com- 

 pleteness, it must be remembered that accuracy of weighing is easily 

 attained. The great errors in chemical quantitative work are usually 

 not in the weighing, but are due rather to impurities in the substances 

 weighed or incompleteness or irregularity of reaction. Only in a few 

 cases, with large vessels, was it necessary to use a telescope for reading 

 the vibrations, the observer being in another room and looking through 

 an intervening glass door. 



The balance-room devoted to these most accurate determinations 

 was kept at a very constant temperature, being wholly inside a private 

 laboratory whose air temperature was regulated by a delicate thermo- 

 static attachment to its heating apparatus. The balance-room itself 

 had neither heating apparatus nor outside windows. This question 

 of constancy of temperature is of the most vital importance in the weigh- 

 ing of large vessels. 



*Proc. Am. Acad., 26, 242 (1891). 



tRichards, J. Am. Cbem. Soc, 22,144 (1900). 



{Richards, Proc. Am. Acad., 31, 175 (1895); Z. Anorg. Chem.,10, 19 (1895). 



