October 27, 1911] 



SCIENCE 



539 



to be weighed must be collected and find its 

 way in due course to the scale-pan. The 

 trouble here lies in the difficulty in esti- 

 mating, or even detecting, minute traces of 

 substances remaining in solution, or minute 

 losses by vaporization at high temperatures. 

 In brief, "the whole truth and nothing 

 but the truth" is the aim. The chemical 

 side of the question is far more intricate 

 and uncertain than the physical operation 

 of weighing. For this reason it is neither 

 necessary nor advisable to use extraordi- 

 narily large amounts of material ; from five 

 to twenty grams in each experiment is usu- 

 ally enough. The exclamation, "What 

 wonderfully fine scales you must have to 

 weigh atoms," indicates lack of knowledge; 

 the real difficulties precede the introduction 

 of the substance into the balance case.* 

 Every substance must be assumed to be 

 impure, every reaction must be assumed to 

 be incomplete, every measurement must be 



Among all the possibilities of error, the 

 unsuspected presence of water is perhaps 

 the most frequent and most insidious. 

 Hence I shall show you a device for over- 

 coming this potent .source of confusion, a 

 device which has played a great role in 

 the recent researches concerning atomic 

 weights at Harvard, and is in large meas- 

 ure responsible for such value as the re- 

 sults may possess. The instrument' en- 

 ables one to dry, enclose and weigh an 

 anhydrous substance in such a manner as 

 to preclude the admission of a trace of 

 water from the atmosphere; it might well 

 find applications in every quantitative 

 laboratory. The simple device consists of 

 a quartz ignition tube fitted to a soft-glass 

 tube which has a projection or pocket in 

 one side (Fig. 1). A weighing-bottle is 

 placed at the end of the latter tube, and its 

 stopper in the pocket. The boat contain- 

 ing the substance to be dried is heated in 



assumed to contain error, until proof to the 

 contrary can be obtained. Only by means 

 of the utmost care, applied with ever- 

 watchful judgment, may the unexpected 

 snares which always lurk in complicated 

 processes be detected and rendered power- 

 less for evil. 



* Richards, "Methods Used in Precise Chemical 

 Investigation," published by the Carnegie Institu- 

 tion of Washington, 1910, No. 125, p. 97. 



the quartz tube, surrounded by an atmos- 

 phere consisting of any desired mixture of 

 •Eichards, Zeitsch. anorg. Chem., 1895, 8, p. 

 267; also Richards and Parker, ibid., 1897, 13, p. 

 86. One form of apparatus shown in this diagram 

 is slightly different from the original arrangement, 

 although the main idea is the same. The flat 

 ground joint between quartz and glass allows for 

 their different coefScients of expansion, and makes 

 a quartz tube interchangeable with any other, in 

 case of breakage. 



