On Mixtures of Hydrochloric Acid and Methyl ether. 593 



wave, starting in the principal focus of a hemispherical mirror, 

 is reflected as a saucer-shaped wave, the curved sides of the 

 saucer coming to a focus in a ring surrounding the nearly 

 flat circular bottom. If we place the lamp in the focus of the 

 mirror, and hold a sheet of ground-glass in front of it at the 

 proper distance, we can show the luminous ring and the uni- 

 formly illuminated circular area within it. If we move the 

 lamp to a point midway between the principal focus and the 

 surface of the mirror, we get a ring of intense brilliancy, with 

 but very little light within it. 



The wave-front constructions for this condition are shown 

 in fig. 3, the distribution of energy being roughly shown by 

 shading the reflected wave-fronts. 



While I have brought out nothing but what would be 

 apparent to anyone on a very cursory examination of the 

 constructions, some of the points may be of use to those 

 engaged in teaching elementary optics. 



University of Wisconsin. 



LVIII. Mixtures of Hydrochloric Acid and Metliyletlier. 

 By J. P. Kuenen*. 



IN 1875 Friedelf discovered that methylether (B. P. 

 — 23°'5 C.) and hydrochloric acid when brought together 

 below zero produce a liquid which boils at 2° C; this liquid 

 was not a chemical compound in the usual sense : the pro- 

 portion in which the two substances are present in the liquid 

 was not always the same, and, moreover, did not correspond 

 to a simple molecular composition (about 6 parts of ether 

 against 5 parts of acid). At the same time the formation 

 of liquid at temperatures far above the boiling-point of the 

 ether and the strong contraction of the vapour-mixture above 

 the boiling-point which he observed proved that the two 

 substances display a strong mutual affinity. He therefore 

 concluded that the molecules of the acid and the ether unite 

 to form a dissociable molecular compound : these double 

 molecules occur in the vapour — hence the abnormally high 

 vapour-density — and especially in the liquid. Both phases 

 are mixtures of double molecules and single molecules of the 

 components. 



From Friedel's observations it follows that mixtures of the 



* Communicated b} r the Author ; being extracted from the Archives 

 Neerlandaises des Sciences exactes et naturellcs. 

 t Compt. Rend, lxxxi. p. 152. 



