AND RATIO OF TIIKIK ATOMIC WEIGHTS. 43 



rests ; the oscillations of the pans and the suspended weights are soon stopped by 

 the friction between the disks and tlie velvet. But when the juui arrests are fur- 

 ther moved down 3.5 centimetres, tlie disks are left perfectly free. 



This apparatus foi* weighing liy reversal was fii'st used with the balance made 

 by Becker, but the a[)paratus for d;nnpiug oscillations was then different. When 

 the Rueprecht balance was put in place of the Becker balance, the ditt'erence in the 

 ari'est for the pans made it necessary to modify the apparatus previously used for 

 this [)ui'pose, and to readjust the distance of the hooks <t and h from the a.\is of the 

 mechanism, to suit the shorter length of the beam of the new l)a!ance. F F ava 

 counterpoises which support the weight of the object carrier, and of the objects 

 placed on it. 



26. OXYGEN HY SECOND METHOD. WETdirTNO BY REVERSAL. 



All the weighings of this series of determinations were made by Gauss' method 

 of reversals. The globe containing the oxygen was suspended from one of the 

 auxiliary pans of the reversal apparatus just described, and its countei-poise from 

 the opposite hook. Weights were placed on the auxiliary pan, on the side from 

 which the globes were suspended (not on the pan of the balance itself), nearly suffi- 

 cient to produce equilibrium. Except in two cases, weights smaller than ten milli- 

 grammes were not used here; as the weights were removed from observation till 

 the end of the operation of weighing, the falling off of a small weight in closing 

 the dooi's of the closet might not be noticed at the time, and it was then a question 

 whether it fell off at the closing of the door at the beginning of the weighing, or at the 

 end. But if a weight of ten milligranunes or more fell oft', there was no ambiguity. 



The globe with its weights, being at the left of the observer, was suspended 

 from the pan of the balance itself by manipulation of the handles shown in Fig. 

 16. The beam of the balance was then released, and the weights smaller than ten 

 milliojrammes which were needed were added to one side or the other as indicated. 

 The balance was then arrested, and the current of dry air introduced into the des- 

 iccator, passing through the axis of the reversing mechanism. After a proper 

 interval, the balance was released and three exeui'sions were noted ; then the revei's- 

 ing mechanism was used to transfer the globe to the right of the observer, to 

 suspend it to the pan, and to stop its vibrations. The small weights" placed on the 

 pan of the balance itself were by hand transferred to the opposite pan, and three 

 excursions were noted. The mean of the apparent weights required in the two 

 positions gives the difference between the weights of the globe and its counterpoise 

 as w^ell as it can be given by a single observation. But in general it will not be 

 the ti'ue difference. 



