218 1'1,'OFKSSOK E. RUTHERFOKD ON THE 



nitrate, obtained from Dr. KNOFFLEK, of Berlin, gave oft' a considerable quantity of 

 radium emanation, led me to examine thorium as well as actinium. 



About 500 grammes of thorium nitrate was taken and dissolved in water. A small 

 quantity of barium nitrate was mixed with it and sufficient sulphuric acid added to 

 precipitate the barium as sulphate. After two successive precipitations all but a 

 small percentage of the radium was removed with the barium. The treated thorium 

 solution was then placed in a closed glass vessel and the emanation allowed to collect 

 in the air-space above the solution. The air was drawn oft' at intervals and passed 

 into a suitable electroscope. The rate of discharge observed was proportional to the 

 amount of emanation present and thus served as a convenient means of determining 

 the variation from time to time of the amount of radium in the solution. 



In a similar way, some of GIESEL'S emanating substance was dissolved in acid and 

 the radium removed by precipitation with barium. The solution has been set aside 

 in a stoppered bottle and the amount of emanation present determined from time 

 to time. 



The observations on actinium have been in progress for three months and for thorium 

 about four months. The earlier observations seemed to indicate a change of the 

 amount of radium present in both the actinium and thorium solutions, but later 

 observations show that if radium is produced at all, it is produced at a very small 

 fraction of the theoretical rate. The experiments on thorium and actinium are being 

 continued, and in the course of a few years it is hoped that a definite answer to the 

 question will be given. 



In experiments of this character certain precautions are essential in order to avoid 

 large possible sources of error in deducing the amount of emanation present by means 

 of the rate of discharge of an electroscope. These sources of error are especially 

 marked in experiments with solutions of thorium or actinium. The emanations from 

 these substances escape into the air-space above the solution. If any dust or water 

 globules are present, excited activity is produced on them. On removal of the air 

 these active carriers are removed with it and unless they are allowed time to settle, 

 or removed by passage through a filter of cotton wool, they are carried into the 

 electroscope and produce an increased rate of discharge. I have found that if the air 

 after removal is stored over water in a gas-holder for an hour the carriers of excited 

 activity are removed from the gas. The effect in the electroscope is then due to the 

 emanation alone. 



No definite experimental evidence is yet forthcoming that radium can be produced 

 by uranium, thorium or actinium. It is not unlikely, however, that the negative 

 results so far obtained may be due to the presence of one or more rayless changes 

 between the parent substance and the radium. We .have seen that such rayless 

 changes exist in radium, thorium and actinium, and the failure to detect the 

 production of radium would be explained if the intervening ' rayless ' products were 

 removed by the same chemical operation which freed the substance of radium. 



