1056 Mr. E. J. Hartung on the Construction and 



theoretical conceptions put forward by Maxwell, and should 

 consequently yield mechanical work at the expense of the 

 kinetic energy of the molecules ; that is of the temperature 

 of the gas as a whole, a result which could not be obtained in 

 any experiment coming within the application of the second 

 law of thermodynamics. 



General experience would indicate, however, that caution 

 is desirable, in accepting this conclusion. 



King's College, University of London, 

 Strand, W.C. 2. 



CXIII. Observations on the Construction and Use of the 

 Steele-Grant Microbalance. By E. J. Hartung*. 



I^HE Steele-Grant microbalance has recently been applied 

 . ' by the writer to the study of the action of light on the 

 halides of silver. During the course of the work some 

 observations have been made on the construction and 

 manipulation of the balance and on possibly unsuspected 

 sources of error. It is hoped that these are of sufficient 

 interest to be given here, as they have not always been empha- 

 sised, even in such an excellent article on microbalances as 

 that of F. Emich (Handb. d. biochem. Arbeitsmethoden, ix. 

 p. 55, 1919). 



The microbalance was described by its inventors in 1909 

 (Proc. Roy. Soc. A. ixxxii. p. 580^ 1909). It is made 

 entirely from vitreous silica and consists of a rigid beam 

 oscillating on a single central knife-edge in the more 

 sensitive forms, or on a pair of edges for greater stability in 

 the less sensitive forms. To one end of the beam a 

 counterpoise of constant weight is fused, to the other a 

 suspended system is attached by a fine silica thread. This 

 suspended system consists of a light silica grid carrying the 

 object to be weighed, as well as certain silica weights and a 

 bulb containing a known quantity of air sealed in it. 

 Equilibrium is attained, at first roughly by selection of the 

 proper weights, and then accurately by adjusting the pressure 

 in the balance case. The constant load is then made up of 

 the object to be weighed, the weights and the apparent 

 weight of! the air-bulb, which varies with the density of the 

 medium in which it is immersed. 



For the investigation mentioned above, a balance of high 

 relative sensitiveness was required. Pettersson (Proc. Phys. 

 Soc. Lond. xxxii. p. 209, 1920) has modified the Steele-Grant 



* Communicated bv the Author. 



