The Laws of Molecular Force. 211 



resistances, but it is not so easily manipulated as the plaster 

 and plumbago. Out of a large number of different substances 

 tried, I find these two the most reliable. A megohm made 

 by the method I have described costs only a few shillings. 



Since October 1888 a D'Arsonval galvanometer has been in 

 constant use here, in combination with a photographic appa- 

 ratus, whereby a cylinder carrying bromide paper is constantly 

 exposed to the reflected light from the galvanometer which 

 shines through a long narrow slit placed in front of the re- 

 volving cylinder ; by this means a constant record is kept of 

 the current in a certain circuit. When the resistance of the 

 galvanometer circuit is very high, the dead-beat action of the 

 instrument is somewhat affected. T find that if a few turns of 

 fine covered copper wire are wound on the outside of the 

 rectangular coil of the instrument so as to form a closed circuit, 

 it gives the same reading as before for a given potential 

 difference, and is perfectly dead-beat in its action, so that 

 with the addition of this damping-coil any amount of re- 

 sistance may be used in the circuit of the galvanometer. 



Trinity College, Oxford, 

 Feb. 10, 1893. 



XXVII. The Laws of Molecular Force. 

 By William Sutherland, M.A., B.Sc* 



IN my last paper on this subject (Phil. Mag. April 1889) 

 it was shown that if the law of force between two similar 

 molecules is 3Am 2 /?* 4 , the parameter A could be calculated 

 for a large number of bodies from Robert Schiff's measure- 

 ments of their surface-tensions, and a law was announced 

 connecting the values of A with chemical composition ; but 

 as this law was affected with exceptions in the case of the 

 organic bromides and iodides and the amines, and as the 

 argument from surface-tension requires the introduction of a 

 considerable number of assumptions, I felt that it was very 

 desirable to secure some other means of finding A. It soon 

 appeared that the only satisfactory plan would be to return 

 again to the search for the true characteristic equation of 

 fluids on the model of Clausius's virial equation, as I had 

 found that not one of those hitherto advanced was capable of 

 general application. Fortunately the experimental material 

 now available is so abundant and so well placed that I was 

 completely successful in the quest, in the more tedious parts 

 of which I had the advantage of assistance from my brother, 

 * Communicated by the Physical Society: read Oct. 28, L892, 



