1862.] 279 



It is only necessary to pass the gas into a watery solution of the 

 chloride of iodine, wash the reddish oil which collects at the bottom 

 of the solution with dilute potash, and distil. The portion which 

 passes over at ahout 145 Cent, is pure chloriodide of ethylene. 



The specific gravity of the chloriodide at zero is 2*151. Heated 

 with an alcoholic solution of potash, it suffers decomposition, iodide 

 of potassium being formed, and a gas given off which burns with a 

 green flame. This is no doubt chloride of aldehydene (C 4 H 3 C1). 

 This reaction goes far to prove that the true constitution of this 

 body is represented by the formula C 4 H 3 C1, HI, and not by the 

 formula C 4 H 3 1, PI Cl, proposed in my former paper. 



Propylene gas derived from glycerine also yields an oil when passed 

 into a solution of chloride of iodine, as I have already stated. In 

 order to purify this, I found it necessary to distil it in vacito, reject- 

 ing what came over at the beginning and towards the end of the 

 process. The numbers I obtained on analysing this body prove its 

 composition to be C 6 H 6 IC1. 



Chloriodide of propylene, as I may call this compound, is when 

 freshly prepared a colourless oil, having an ethereal odour and a sweet 

 taste. Its specific gravity at zero is T932. "When an effort is made 

 to distil it under atmospheric pressure, it suffers decomposition, hy- 

 driodic acid being evolved in large quantity. Mixed with an alcoholic 

 solution of potash and distilled, it yields iodide of potassium and an 

 oily liquid (contained in the distillate and separable from it by water) 

 which is very volatile and burns with a green flame. This is doubt- 

 less chloride of allyle (C 6 H. Cl). 



The oil formed by the action of chloride of iodine on propylene 

 gas obtained from amylic alcohol, I have not been able to obtain in 

 a fit state for analysis. 



The application of the foregoing process to other hydrocarbons 

 would no doubt place in our hands many similar compounds. 



V. " On certain Developable Surfaces." By A. CAYLEY, Esq. 

 Received October 25, 1862. Read November 27, 1862. 

 (Abstract.) 



If IJ=0 be the equation of a developable surface, or say a develop- 

 able, then the hessian HU vanishes, not identically, but only by virtue 

 of the equation U=0 of the surface; that is, HU contains U as a 



