FUNDAMENTAL. PROPERTIES OF THE ELEMENTS RICHARDS. 201 



include and occlude solvents, precipitates carry down polluting 

 impurities, dried substances cling to water, and solids, even at high 

 temperatures, often fail to discharge their imprisoned contaminations. 



In the next place, after an analysis has once begun, every trace of 

 each substance 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 estimating, or even detecting, minute traces of substances remain- 

 ing in solution, or minute losses by evaporation 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 extraordinarily large amounts of mate- 

 rial. From 5 to 20 grams in each experiment is usually 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. 1 Every 

 substance must be assumed to be impure, every reaction must be 

 assumed to be incomplete, every measurement must be 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 powerless for evil. 



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 overcoming this potent source of confusion, a 

 device which has played a great role in the recent researches concern- 

 ing atomic weights at Harvard, and is in large measure responsible 

 for such value as the results may possess. The instrument 2 enables 

 one to dry, inclose, 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 containing the substance to 

 be dried is heated in the quartz tube, surrounded by an atmosphere 

 consisting of any desired mixture of gases. These gases are dis- 

 placed after partial cooling, first by nitrogen and then by pure dry 

 air, and the boat is pushed past the stopper into the weighing bottle, 



1 Richards, Methods Used in Precise Chemical Investigation, published by the Carnegie Institution of 

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



* Richards, Zeitsch. anorg. Chem., 1895, vol. 8, p. 267; also Richards and Parker, ibid., 1897, vol. 13, p. 86. 

 Two forms of apparatus are shown in this diagram; the upper drawing depicts the earlier form, suitable for 

 a hard glass or porcelain ignition tube, whereas the lower drawing illustrates a form 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 coefficients of expansion, and makes a quartz tube interchangeable with any 

 other in case of breakage. 



