608 HEPORT— 1887. 



TUESDAY, SEPTEMBER 6. 



The following Papers and Report were read : — 



1. On the Magnetic, Properties of Oases. By Professor Qdincke. 

 The magnetic pressure on the unit of area in a body is 



wliere Hj is the strength of the magnetic field. We can compare the diiferenee of 

 the magnetic pressures of two different substances at their common surface with a 

 hydrostatic pressiu'e. This is done for a liquid and a gas by the magnetic 

 manometer — a U tube, with two branches of different diameter, filled with the 

 liquid. The surface of the liquid in the smaller branch is brought into the magnetic 

 field of a powerful electro-magnet ; the other branch remains in a field of constant 

 strength. The change h of the height of the liquid with the specific gravity a is 

 3Qaeasured, and we have the hydrostatic pressure 



where .R and Jtj^are the diamagnetic constants of the liquid and of atmospheric air. 



Dr. Quincke compared in this way, some years ago, different liquids with 

 common air, and has now compared the same liquid (petroleum, alcohol, water) 

 with different gases of different density. 



The change of the hj^drostatic pressure increases nearly proportionately with 

 the density of the gases. If we assume that the qualities of the liquid are not 

 changed by the absorbed gas, we can find from the difference of the changes of the 

 hj'drostatic pi'essure, divided by the difference of the densities of the gas, the 

 magnetic pressure of the gas for one atmosphere, or the diamagnetic constant St in 

 absolute measure for any gas at noi-mal pressure and ordinary temperature. 



The gases were compressed by an ordinary compressing pump, with a fly-wheel : 

 the density was measured by an air-manometer, consisting of a horizontal 

 thei'mometer-tube, closed at one end, containing air and a thread of mercury. The 

 pressure did not exceed 40 atmospheres. The numbers are given in the O.G.S. 

 system : — 



Oxygen . 

 Nitric Oxide 

 Air 



Olefiant Gas 

 Carbonic Acid 

 Marsh Gas 

 Nitrogen 

 Hydrogen 

 Vacuum . 



These results agree with Faraday's relative values (Faradav's 'Exp.,' sec. iii. 

 p. 502). 



The diamagnetic constant of a perfect vacuum cannot be found by this method ; 

 but only the difference of the diamagnetic constants of a vacuum and the liquid in 

 the magnetic manometer. 



2. Report of the Committee for constructing and issuing Practical Standards 

 for use in Electrical Measurements. — See Reports, p. 206. 



3. On the Permanence of the B.A. Standards of Resistance. 

 By R. T. Glazebrook, F.R.S. 



