384 



Dr. W. Marcet. 



[June 21, 



of the volume after explosion. It will be seen that these conditions 

 are entirely fulfilled in the present instrument ; and it has, moreover, 

 the advantage of being available in conjunction with Pettenkofer's 

 method for the determination of carbonic acid in atmospheric 

 air. 



The eudiometer as figured in the accompanying Plate has the 

 form of a *J"-piece, the vertical limb of which is a straight tube about 

 60 cm. in length and 12 cm. in diameter ; it is divided into 50 or 60 c.c. 

 and tenths of c.c, like a common burette. The upper end of tins 

 tube is closed air-tight with a steel cap, from which lateral tubes 

 project right and left ; these tubes are bent \/-shaped, or rather in the 

 form of a lyre. At the junction of the lateral tubes with the cap, 

 there is a three-way stop-cock allowing of the passage of air or gas 

 in four different directions, viz., first through the tubes cut off from 

 the body of the eudiometer ; secondly, into the eudiometer, which is 

 done by raising it in the mercury trough ; thirdly, out of the eudio- 

 meter, on the side opposite that from which it was introduced, which 

 is effected by depressing the tube in the mercury ; fourthly, through 

 the tubes and eudiometer simultaneously. The eudiometer is held 

 tightly by two claws projecting at different heights from a vertical 

 iron rod connected with a rack and pinion movement. The iron rod, 

 together with the eudiometer, is immersed in mercury contained 

 in a straight cylindrical glass vessel. 



The hydrogen used for the explosion is prepared for that special 

 object from zinc and sulphuric acid in the ordinary way, and washed 

 through an alkaline solution, rather than obtained condensed in iron 

 bottles from the manufacturers, and it is collected in a bell-jar 

 suspended over water. The bell -jar I use holds 11 litres of gas ; it is 

 balanced by a counterpoise, and its weight, as it moves up and down 

 in water, is regulated by another counterpoise hanging from a cycloid, 

 so that the gas in the holder is always under atmospheric pressure ; 

 an oil-gauge fixed to the holder shows at any time the pressure in the 

 bell-jar. Should the gas fail to be absolutely under atmospheric 

 jDressure, the equality of pressures may be ensured by the use of the 

 adjusting instrument I have described in a former communication. 

 It consists of a clamp fixed to the rim of the tank, and made to grasp 

 at will the cord holding the counterpoise ; a screw in connexion with 

 the clamp enables the cord, and consequently the bell-jar, to be 

 drawn up or down. For the actual requirements of the analysis, a 

 receiver for the hydrogen holding only one litre of gas would suffice, 

 but it is better to have a larger gas-holder in which to store up the 

 hydrogen for future determinations. 



Moreover, the cycloid arrangement for regulating the weight of the 

 bell- jar, though very convenient, may be dispensed with, as the gas in 

 the receiver can be brought approximately under atmospheric pressure 



