RECENT STUDIES IN GRAVITATION. 213 



vibration of more than 1 second of are, while the disturbances were 

 sometimes 50 times as great. 



The semicircular couple required the turning sphere to revolve in 

 one hundred and fifteen seconds. Here want of symmetry in the 

 apparatus would come in with the same effect as the couple sought, 

 and the outstanding result was accordingly a little larger. 



But in neither case could the experiments be taken as showing a real 

 couple. They only showed that, if it existed, it was incapable of pro- 

 ducing an effect greater than that observed. 



Perhaps the best way to put the result of our work is this: Imagine 

 the small sphere set with its axis at 45° to that of the other. Then 

 the couple is not greater than one which would take five and one-fourth 

 hours to turn it through that 45° to the parallel position, and it would 

 oscillate about that position in not less than twenty-one hours. 



The semicircular couple is not greater than one which would turn 

 from crossed to parallel position in four and one-half hours, and it would 

 oscillate about that position in not less than seventeen hours. 



Or, if the gravitation is less in the crossed than in the parallel posi- 

 tion, and in a constant ratio, the difference is less than 1 in 16,000 in 

 the one case and less than 1 in 2,800 in the other. 



We may compare with these numbers the difference of rate of travel 

 of yellow T light through a quartz crystal along the axis and perpen- 

 dicular to it. That difference is of quite another order, being about 1 

 in 170. 



As to other possible qualities of gravitation, I shall only mention 

 that quite indecisive experiments have been made to seek for an altera- 

 tion of mass on chemical combination, 11 and that at present there is no 

 reason to suppose that temperature affects gravitation. Indeed, as to 

 temperature effect, the agreement of weight methods and volume 

 methods of measuring expansion with rise of temperature is good, as 

 far as it goes, in showing that weight is independent of temperature. 



So, while the experiments to determine G are converging on the 

 same value, the attempts to show that, under certain conditions, it may 

 not be constant, have resulted so far in failure all along the line. No 

 attack on gravitation has succeeded in showing that it is related to any- 

 thing but the masses of the attracting and the attracted bodies. It 

 appears to have no relation to physical or chemical condition of the 

 acting masses or to the intervening medium. 



Perhaps we have been led astray b} r false analogies in some of our 

 questions. Some of the qualities we have sought and failed to find, 

 qualities which characterize electric and magnetic forces, may be due 

 to the polarity, the + and — , which we ascribe to poles and charges, 

 and which have no counterpart in mass. 



■ Landolt, Zeit. fur Phys. Chem. XII, 1, 1894. Sanford and Ray, Physical Review, 

 V, 1897, p. 247. 



