546 



Hence we must conclude, within the limits of experimental errors, 

 that if energy possesses mass it also possesses weight. Now we can 

 considerably restrict these limits by the use of the torsion balance 

 and this justifies us, I think, in applying it to the investigation of 

 radioactive substances, 



5. New experiments. My own experiments were made with an 

 apparatus, principally after the design of that of v. Eötvös, but of 

 much smaller dimensions. The weights at the end of the torsion 

 rod were each 30 grams in v. Eötvös' experiments ; in my apparatus 

 the weights were each of 1 gram. The weight of the torsion rod 

 with mirror was only about 1,5 grams. The distance between the 

 centres of the cylindrical weights at the ends of the torsion rod is 

 about 10 cms. The smallness of these weights enabled me to take 

 advantage from the properties of fine quartz wires, not yet discovered, 

 indeed, at the time v. Eötvös began his researches. 



The torsion wire in my apparatus was22cms.long,and abontO,Ol mm. 

 thick. The time of oscillation ranged from about 350 to 400 seconds. 



In order to protect the apparatus from thermal and electrical 

 perturbations we used, as also did v. Eötvös, double and even triple 

 walls, of brass, about 3 mms. thick. 



Manipulations with an apparatus of so great a sensibility as this 

 torsion balance, requires exceptional stability of the surroundings. 

 The mounting on the brick piers of the Amsterdam laboratory proved 

 to be quite insufficient. I, therefore, resolved to construct an arrange- 

 ment, probably securing tlie apparatus against vibrations and per- 

 mitting its rotation, with scale and telescope, about a vertical axis 

 through 180°. 



The principle of this arrangement is the one used by Michelson 

 and by Einthoven for similar purposes. 



I have much pleasure to thank here Mi-. W. M. Kok, phil. nat. 

 cand. for his assistence in the construction of this arrangement and 

 during the continuation of the present investigation. 



The principal part of the arrangement is an iron basin floating 

 in a tank with thick oil. Tank and basin are of annular form ; the 

 central part being open, it becomes possible to suspend an apparatus 

 from a vertical bar through the centre of the annular basin and 

 to fix it at different heights. 



A more detailed description is reserved for another occasion. 



It was found that this arrangemenc gave excellent protection against 

 vibrations of short period and permitted» also to give slow and 

 smooth rotations. 



