66 



Mr. J. Stuart on Galvanomagnetic Attraction. [Dec. 5, 



strongly heated ; they melt, boil, and yield a semisolid distillate. The distil- 

 lation is carried on as far as possible without charring the residue, which 

 consists of potassium benzoate (from which half the benzoic acid employed 

 may be recovered). By addition of ammonia to the semisolid distillate 

 any benzoic acid that has passed over is retained, whilst the nitrile distils 

 with the water, from which it is afterwards separated, dried, and re- 

 distilled. A roughly carried out experiment yielded 50 per cent, of the 

 theoretical quantity of nitrile in a perfectly pure condition ; the loss owing 

 to secondary reactions is amply compensated by the ease and rapidity of 

 the operation. 



Action of Cuminic Acid on Potassium Sulphocyanate. 



An experiment with cuminic acid yielded very satisfactory results : 

 cumonitrile was obtained in about the same proportions as the benzo- 

 nitrile. The temperature at which the reaction commenced was here about 

 210° ; the nitrile was purified as in the preceding case. 



Finally, an experiment was made with cinnamic acid ; but although 

 sulphuretted hydrogen was evolved, and the reaction appeared to proceed 

 exactly as in the foregoing cases, no cinnamonitrile was obtained in the 

 liquid disillate. The cinnamic acid seemed to be decomposed into carbonic 

 anhydride and cinnamol before being acted upon by the sulphocyanic 

 acid. 



The ease with which nitriles and amides are obtained in this manner, 

 both in the aromatic and fatty series, induces me to hope that the new 

 method may be of use in many cases, and perhaps, by its application 

 to other series, give rise to bodies hitherto uninvestigated. 



The tediousness of preparing the acid chloride and subsequent treat- 

 ment with ammonia (for the amide), or of the amide with phosphoric 

 anhydride (for the nitrile), in the ordinary method for producing these 

 nitrogen compounds, is here replaced by simple digestion of the acid with 

 sulphocyanate of potassium, bodies generally readily procurable. 



V. " Investigation of the Attraction of a Galvanic Coil on a small 

 Magnetic Mass." By James Stuart, M.A., Fellow of Trinity 

 College, Cambridge. Communicated by the President. Received 

 July 26, 1872. 



From investigations given by Ampere, we can deduce an expression 

 for the potential TJ at an external point Q of a closed circular galvanic 

 current carried by a wire of indefinitely small section. Let a be the 

 radius of the circle ; let the distance of Q from C, the centre of the circle, 

 be r ; and let the line C Q make an angle 6 with the normal to the plane 

 of the circle. Then it can be shown that when r is less than a, 



