308 



Prof. J. C. Ewart. 



[June 14, 



Table XIX. — Minimum Proportions. 



Iodine, between 1 part in 3,100,000 and 3,521,970 

 Bromine „ 1 „ 77,500,000 „ 84,545,000 



Chlorine „ 1 „ 1264,000,000 „ 1300,000,000 



This series of numbers suggests a quantitative relation of the 

 "minimum proportions " to the atomic and molecular weights of the 

 substances. 



On comparing these numbers with those of the two previous 

 groups of bodies, we find that the proportion of substance required to 

 upset the voltaic balance was largest with the oxygen salts, inter- 

 mediate with the haloid ones, and least with the free elementary 

 bodies. It was smaller the greater the degree of chemical energy of 

 the substance ; thus it was about 400 times less with chlorine than 

 with iodine. And it was smaller the greater the degree of freedom 

 to exert that energy ; thus it was about 5,416,000 times smaller with 

 free chlorine than with potassic chlorate, or 1,570,000 times less than 

 with the combined chlorine of the chlorate; and about 185 times 

 smaller than with potassic chloride, or 88 times less than with the 

 combined chlorine of that salt. 



At the lowest potentials, the rate of increase of electromotive force 

 per grain of substance is usually larger the smaller the proportion of 

 substance necessary to disturb the potential. Iodine is an exception 

 to this, but probably only an apparent one, because on substituting 

 magnesium for the zinc, the addition of iodine caused an increase of 

 potential as usual. 



The curve of variation of potential was different with the solution 

 of each substance, and was apparently characteristic of the body in 

 each case ; and a great number of such representative curves might 

 be obtained by change of strength of solution, in nearly all electrolytes, 

 with a zinc-platinum or other voltaic couple. 



IV. " The Electric Organ of the Skate. The Electric Organ of 

 Raia radiata" By J. C. EWART, M.D., Regius Professor of 

 Natural History, University of Edinburgh. Communicated 

 by Professor J. Burdon Sanderson, F.R.S. Received 

 June 6, 1888. 



(Abstract.) 



The first part of this paper is chiefly devoted to a comparison of 

 the electric organs of Raia radiata, R. batis, and R. circularis. It is 

 shown that the organ in the species radiata differs in many respects 

 from the organ in the two other species, and that an exhaustive 



