The daily morning journal and courier., 1899-12-28
ADDRESS BY MR. MORLEY.
Are Nrw Processes Needed for the De- termination of the Atomic Weighf of OxVpeut

l Are NfW Processes Needed for the Dee | termination of the Atomic Weighf of Oxygen.
 | The ratio of the atomic weights of
 oxygen have been determined by many
 experimenters, and the mean error of
 each experiment. has been gradually so
 much reduced that in some cases it has | become less than one part in ten thou | sand. The results of different experi- l mentors differ much more thn this i from each other. But even if the re- | suits of different eXperimenters agreed | closely, the question, how accurately we know the ratio sought, is II one which we readily answer. Neither the con- cordance of individual results of the same series, nor the agreement of the | results of the different series, can es- tablish beyond question the accuracy of a quanttative determination All
 the sciences which have to do with measurement afford many instances of
 this, and chemistry furnishes examples
 of the fact that most of our conclusions
 are to be received with certain sus-
 pense of judgment, that all measure ments are but approaches to something
 which can never be quite reached, and
 that he is wise who accepts all asser-
 tions only until there is evidence to the
 contrary.
 An interesting case of this kind, where neither concordance of individu- al results, nor agreement of results of different experimenters, was a suffi- cient guarantee of accuracy, may be gound in the determina tions of the atomic weight of oxygen which were
 made about the middle of the century. Dumas obtained the value 15.96 for the ratio sought; the mean error of an ex- pertinent was only one part in four hundred. The number of experiments was nineteen, and the probable error of the mean of all the experiments was therefore only one part in twenty-two hundred fifty. Accordingly, it was much more than an even chance that the second decimal figure of Dumas re- suit would not have been much changed | by the accumulation of even a very large number of experiments made in ! the same way, with the same skill and | care But as to the difference between | this result of the nineteen experiments and the unknown true value. we are | told absolutely nothing by the propos- !
 
 result was one part in twenty-two hundred fifty. It is commonplace to say that the cauculation of the proba- ble error of series of experiments f does not show how nearly the result ap proaches the truth, but how near it is to the result of a greater number of | similar experiments It decides, not how nearly we approach the desired goal but whether it is useful to per severe by the present method of ap- proach. Dumas made nineteen obser vations, and got the value 15.96 with a probable error of 0.007; that is, if he had made a hundred or ath ousand eX- periments it is unlikely that the final result would not have been between 15.95 and 1597, and very unlikely in- deed that it would not have been be- tween 15.94 and 15.9S. But he would never have obtained a value near that which now commands confidence
 It is interesting to recall that there is hardly any instance on record where the judgment of an eXperimenter as to the degree of approximation to the truth attained in his work has been better justified than in the case of Du- mas classic experiments. As We all towards the end of his work, there was discovered in his own Iabo- ra tory a source of error, not easy to eliminate which had affected all his de- termination. The account of the error was not fixed quantity, and no nu-