1888.] 



Densities of Hydrogen and Oxygen. 



357 



conducted with gases under the normal atmospheric conditions as to 

 temperature and pressure, any small departures from the laws of 

 Boyle and Charles will be practically without influence upon the final 

 number representing the ratio of atomic weights. 



In weighing the gas the procedure of Regnault was adopted, the 

 working globe being compensated by a similar closed globe of the same 

 external volume, made of the same kind of glass, and of nearly the 

 same weight. In this way the weighings are rendered independent of 

 the atmospheric conditions, and only small weights are required. The 

 weight of the globe used in the experiments here to be described was 

 about 200 grams, and the contents were about 1800 c.c. 



The balance is by Oertling, and readings with successive release- 

 ments of the beam and pans, but without removal of the globes, 

 usually agreed to -roth mgrm. Each recorded weighing is the mean 

 of the results of several releasements. 



The balance was situated in a cellar, where temperature was very 

 constant, but at certain times the air currents, described by Professor 

 Cooke, were very plainly noticeable. The beam left swinging over 

 night would be found still in motion when the weighings were com- 

 menced on the following morning. At other times these currents 

 were absent, and the beam would settle down to almost absolute rest. 

 This difference of behaviour was found to depend upon the distribution 

 of temperature at various levels in the room. A delicate thermopile 

 with reflecting cones was arranged so that one cone pointed towards 

 the ceiling and the other to the floor. When the galvanometer indi- 

 cated that the ceiling was the warmer, the balance behaved well, and 

 vice versa. The reason is of course that air is stable when the tem- 

 perature increases upwards, and unstable when heat is communicated 

 below. During the winter months the ground was usually warmer 

 than the rest of the room, and air currents developed themselves in 

 tbe weighing closet. During the summer the air cooled by contact 

 with the ground remained as a layer below, and the balance was 

 undisturbed. 



The principal difference to be noted between my arrangements and 

 those of Professor Cooke is that in my case no desiccators were used 

 within the weighing closet. The general air of the room was pre- 

 vented from getting too damp by means of a large blanket, occa- 

 sionally removed and dried before a fire.* 



In Regnault's experiments the globe was filled with gas to the 

 atmospheric pressure (determined by an independent barometer), and 

 the temperature was maintained at zero by a bath of ice. The use of 

 ice is no doubt to be recommended in the case of the heavier gases ; 

 but it involves a cleaning of the globe, and therefore diminishes some- 



* I can strongly recommend this method. In twenty-four hours the blanket will 

 frequently absorb two pounds of moisture. 



