( 109 ) 



The manometer jacket was supplied trom underneath witli water 

 from the thermostat, (see § 5). From above the water flowed out 

 through a eoimecting tul)e which was well protected from heat 

 exchange with the surroundings to the lower portion of the volumeno- 

 meter jacket. Hence variations in the temperature of the jacket water 

 occasion errors which neutralise each other at least in part, a result 

 which one cannot be sure of obtaining when the water supplies are 

 independent or form branches of one circuit. 



In the room in which readings are taken it is difficult to keep 

 the temperature gradient below ^1° per metre ; it often assumes 

 greater values and, in consequence, it is easy for the mercury in the 

 bulb and in the rubber tubes to assume a temperature different from 

 that obtaining in the mercury columns within the jackets; on this 

 account the water which flows out from the upper end of the 

 volumenometer jacket is utilised to warm or cool as the case may 

 be the mercury which flows from the mercury bulb to the mano- 

 meter or to the volumenometer. For this purpose the short rubber 

 tubes are joined to the large glass T-piece. The vertical arm of this 

 T-piece is widened so as to obtain a greater surface for heat exchange, 

 and this vertical arm is surrounded by the two copper tubes shown 

 in PI. I, fig. J. The water which is flowing off from the volumeno- 

 meter is then divided into two circuits which traverse these copper 

 tubes and then leave the apparatus at I^ and I^. By this arrangement 

 are eliminated all undesirable convection currents and conduction 

 phenomena in the masses of mercury. 



In conclusion we may mention that all parts of the apparatus 

 were carefully wrapped up so as to be protected from heat exchange 

 with the outside. 



After these precautions had been taken, a great number of mea- 

 surements were made of the heights of the menisci in the communi- 

 cating vessels. The capillary depressions were obtained from Kelvin's 

 graphical constructions in which a mean capillary constant was 

 assumed. After a proper application of the temperature and capillary 

 corrections the optical corrections were calculated as the remaining 

 difference in height. In Table II are collected the mean data obtained 

 from the various measurements which with the help of the method 

 of communicating vessels have led to one of tlie determinations of 

 the optical constant for the mark m^. The means of all the mea- 

 surements are given in Table III, col. IV. 



Since there can be no doubt about the pressure equilibrium a 

 comparison of the optical corrections thus determined with those 

 obtained by the method of the combination level with the removal 



