726 



quite aocuratelj in 5 lioiirs, among whicli occur several measurements 

 which vvei-e re|)eated sometimes for the sake of a greater certainty 'J. 

 Nor was it necessary for me to glow the substance before measuring 

 its ionisation. Prof. Gockkl, namely, expei'ienced the disadvantage 

 that during the hour of the measurement emanation escaped, the 

 radio-active products of which settled on the sides of the vessel, thus 

 causing the ionisation to grow stronger during the measurement; 

 as several circumstances, a. o. the nature of the substance, are of 

 great intluence on this so-called "emanating", Gockel ti-ies to avoid 

 this difiiculty by glowing the substance beforehand, or: to deprive 

 it of all emanation present. This is an uncertain procedure, which 

 BücHNER has already pointed out (cf. note p. 725) and I think it is 

 also preferable not to shut out its activity, if the substance should 

 contain some emanation. This is possible if the measurement is but 

 short, as in that time the quantity of escaped emanation need not 

 be taken into account. In my opinion both methods have great 

 value and the importance of the "direct method" should not 

 be underrated; first because of the advantages mentioned in order to 

 determine the order of radio-activity, as in the case of the samples 

 of Part II, by a speedy inquiry, but secondly to penetrate deeper 

 into the radio-active phenomena of the substances itself. It will 

 never be possible to say beforehand what radio-active products, 

 perhaps new ones, one may happen to meet, or whether, as Prof. 

 GocKEL also remarks, all the products are indeed present, which 

 arise from a series of active substances the emanation of which is 

 found according to the emanation method. With the help of absorption 

 experiments and especially in very accurate measurements it seems 

 to me that especially in the future this method may become of 

 veiy great value. 



In order to complete the above brief description of the method, 

 I may add the following: first of all the placing of the samples 

 into the ionisation space was done in a particular way. Prof. Gockel 

 describes the difficulty occurring in his case, viz: that in the space 

 of experimentation a fresh supply of air will always penetrate, which 

 in itself will modify the ionisation. In order to avoid this disturbance 

 the electrometer employed by me was provided with a special 



1) It is important to remark here that a sensitive measuring method, "direct" 

 as well as indirect, possesses advantages over an insensitive one ; where up to now, 

 many investigators use the latter, this will explain, though only partly, the 

 remarkable fact that as regards the measuring results of various observers one 

 meets with so much contradiction, where determinations of the radio-activity of 

 similar material are concerned. 



