294 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In the compilation of tlie following table the chief emphasis has been 

 laid on the probable chemical accuracy of the processes involved in each 

 case. The search for possible causes of constant chemical error has 

 usually furnished the evidence, if any evidence was to be obtained, upon 

 which has been based the sometimes very puzzling decisions between 

 conflicting data. For example, the possible occlusion of foreign matters 

 by many precipitates has always been borne in mind, as well as the fact 

 that crystals obtained from aqueous solutions usually retain with great 

 obstinacy important amounts of mother liquor enclosed within their 

 structure. Oxides and other substances obtained by ignition often have 

 a tendency to occlude traces of other materials remaining from their 

 mother compound ; of course the possibilities of such a serious error 

 must be carefully weighed in each case. Most experimenters upon 

 the volumetric ratio of silver to the chlorides have produced erroneous 

 data, through lack of knowledge of the dangers of this apparently sim- 

 ple process. These results must then be given small weight, no matter 

 how concordant they may seem. The solubility of glass and porcelain, 

 as well as of nearly all precipitates, is another frequently overlooked 

 source of grave complications, and so is the action of many substances 

 upon their containing vessels at high temperatures. The fact that in 

 some cases the necessary methods of calculation greatly magnify the 

 experimental error was not forgotten. These and many other equally 

 obvious minor principles have served as the guides in the selection of 

 the figures below ; but of course the results cannot be considered ulti- 

 mate. " Post-mortem examinations are often unable to detect the more 

 subtle poisons." In many cases much more experimental work is 

 needed, not only to secure new data, but also to show where the old 

 are in error. While, however, it is very important to show such falsity, 

 the attempt to counteract it numerically by the api)lication of small 

 corrections is a dangerous remedy ; for minor conditions often seriously 

 modify the minor corrections. In such cases wholly new data must 

 usually be obtained, and these must be corrected, if correction is neces- 

 sary, by him who is familiar with every step of the processes involved. 



One is continually reminded, as he studies the conflicting evidence, 

 of the words of Ostwald : " DifFerenzen sind nicht selten, und mahnen 

 ernstlich an die Unvollkommenheit menschlicher Arbeit, aber audi an 

 die Verantwortung, welcher jeder iibernimmt, welcher einen numerischeu 

 Wert der Oeffentlichkeit mitteilt." * This responsibility is not always 



* Zeit. phys. Chem., III. 14-3. 



