632 REPORT— 1903. 



notewortliy that the velocity of the reaction hetween mercuric chloride and potas- 

 sium iodide is enormously greater than that between lead chloride and potassium 

 iodide when dried in the same way. The factors to which this difference may be 

 referred are (1) solubility, (2) volatility, (3) degree of ionisation, (4) specific" re- 

 action velocity. We will consider these in order. 



(1) Mercuric chloride is about ten times as soluble as lead chloride in cold water, 

 but this alone would not account for the difference ; e.g. mercuric cyanide is still 

 more soluble, but does not react at all. 



(2) Judging from the boiling-points, mercuric chloride would appear to be 

 more volatile than lead chloride, the boiling-point of the former being 300° 0. and 

 that of the latter about 900° C. ; but I find that on aspirating air over each salt 

 and then over potassium iodide, the vapour from the lead chloride affects the 

 potassium iodide much the sooner. 



The difference in the speed of the two reactions {a and c) cannot therefore be 

 caused by the difference in volatility. 



(3) The degree of ionisation cannot be the cause of the difference noted, for 

 mercuric chloride is known to be very slightly ionised in solution, while lead 

 chloride may be taken as completely ionised. 



(4) The specific reaction velocity appears to be the real determining factor, and 

 the reaction is probably of the form AB + CD = AD + BC. If it is only free ions 

 that react (which seems to me improbable), then the velocity of ionisation in the 

 case of mercuric chloride must be extremely great. 



There may also be other factors not yet understood. 



6. Report of the Committee on the relation hetween the Absorption Spectra 

 and Chemical Constitution of Organic Substances. — See Reports, 

 p. 126. 



TUESDAY, SEPTEMBER 15. 

 The following Papers and Reports were read : — 



1. Freezinq-point Ciirres for Binary Systems. 

 By James C. Philip, M.A., Ph.D. 



When a liquid mixture of two components is cooled, a point is reached at 

 which separation of solid takes place. For complete interpretation of the pheno- 

 mena, it is necessary to know not only (1) this temperature of initial freezing, 

 but also (2) the composition of the separating solid, and (3) that of the liquid 

 from -which it separates. The varying character of the relation between (2) and 

 (3) is best demonstrated graphically by plotting the one against the other in a 

 square diagram. It is then found that the cases experimentally known fall into 

 one or other of two classes, according as the composition of the solid varies con- 

 tinuously with that of the liquid, or is constant for certain ranges of concentration, 

 and to that extent independent of the composition of the liquid. To the former 

 class belong systems of two components that form mixed crystals ; the components 

 of systems in the latter class do not form mixed crystals, and the definite solids 

 that separate out, each within its own range, are either the pure components or 

 compounds of these. If consideration is confined to the latter class of cases, it is 

 found that on the freezing-point curves (i.e. the curves obtained by plotting the 

 temperature of initial freezing against the composition of the liquid) there is a 

 branch corresponding with each range of concentration over which the separating 

 solid is definite and constant. With the intersection of two branches on the 

 freezing-point curve, there corresponds a vertical line on the square diagram, and 

 •where the freezing-point curve has an intermediate branch with a summit, there ia 



