538 Scientific Proceedings, Royal Dublin Society. 



in the construction by conferring an equally small area on d, but it 

 is probably more convenient to make the simple correction re- 

 quired. The barometer is now read, and the pressure of the gas 

 in g computed. Its temperature is 0°C, and its volume that of the 

 vessel = V. The sphere is now returned to the balance, re- weighed, 

 and the weight of gas, subject to a small correction, found by 

 difference. 



The correction mentioned is due to the shrinkage of the copper 

 sphere on the relief of pressure, and corresponds to that occurring 

 in the case of Eegnault's glass vessel. Experiments (two, closely 

 agreeing, have been made on this sphere) reveal a shrinkage 

 which may be taken as 0-1732 c.cs. on the removal of 4*3525 

 grammes of air at the temperature 12°C, a fall of pressure of 22-01 

 atmospheres, closely. At 760 mms. this weight of air occupies 

 about 3-367 litres. This would be a needlessly large volume to 

 confer on the volumenometer, except for very special work- 

 Even in the case of hydrogen (1 litre at 760 and 0°C, weighing 

 0-0895 grams.) two litres will suffice for a very accurate estimation 

 of the weight of gas dealt with. At the pressure corresponding 

 to the compression of two litres into the sphere of 160 c.cs. 

 capacity, a much lighter weighing sphere than that previously 

 described will suffice. Thus I use one weighing but 41 grammes, 

 which has been tested up to 500 lbs. in the square inch. The 

 thickness of its walls is somewhat less than half a millimetre, its 

 volumes a little greater than 160 c.cs. There is no difficulty in 

 weighing such a vessel to the one-twentieth of a m.gram., or 

 closer. Using the heavier vessel the correction for shrinkage will 

 be the shrinkage due to two litres, i. e. 0-1029 c.cs. multiplied into 

 the normal density of air. This gives a displacement effect of 

 0-000127. This correction is on a weight of 2-5878 grams, of air 

 or the correction on w, which is subtractive, is for air one part in 

 25,000 g.p. In the case of hydrogen one part in 1790. Assum- 

 ing the use of the lighter sphere the experimentally determined 

 shrinkage is 0-26359 c.cs. for a lowering of pressure of 12-S2 

 atmospheres, a little more than the pressure due to two litres. 

 The shrinkage is, in fact, closely 0-257 c.cs., and the correction on 

 the weight of gas released is 0-000318. This is higher than with 

 the heavier sphere, but the correction is perfectly definite [and 

 easily ascertained once for all by successive weighings, before and 



