608 KEPOET— 1891. 



stannic chloride, gold chloride, and gold. The E.M.F. is found to rise abruptly on 

 passing from alloys containing 36 per cent, to those containing 38 per cent, of tin, 

 thus indicating the existence of a compound of the formula AuSn. This agrees 

 with the maximum point in Matthiessen's conductivity curve for these alloys. 

 Some preliminary experiments with gold aluminium alloys show that there is a 

 rise in E.M.F. on passing over Professor Roberts-Austen's new purple alloy. This 

 method not only indicates the existence of a compound, but also enables us to cal- 

 culate approximately the heat of formation of the compound from the rise in 

 E.M.F. in passing from the alloys below to those above the compound. 



3. On the delation between the Composition of a Double Salt and the Gom- 

 position and Temperature of the Solution in ivhich it is formed. By 

 A. Vernon Haucoort, F.B.S., and F. W. Humphery, of Christ 

 Church, Oxford. 



The particular double salt of which the authors have prepared and analysed 

 some seventy specimens is ferrous and ammonium chloride. Probably many other 

 double salts, if similarly examined, would show similar variations. When ferrous 

 chloride and ammonium chloride are dissolved together in warm water, and the 

 saturated liquid is allowed to cool, white crystals are deposited, whose composition 

 varies with the proportion of the two salts in solution and with the temperature at 

 which the crystals are formed. Since the proportion of the two chlorides in the 

 liquid and in the crystals which form in the liquid is very diflerent, the compo- 

 sition of the liquid changes continually during crystallisation, and no two portions 

 of the double salt are formed under exactly the same conditions. In the authors' 

 later experiments the variation from the begianing to the. end of a crystallisation 

 was reduced to about 2 per cent, by taking a crop of crystals weighing only eight 

 or ten grams from a liquid containing some hundreds of gTams of each chloride. 

 By the use of a water-bath, in which the flask holding the solution was plunged, 

 warmed by a gas-burner which was govei-ned by a thermostat in the liquid, the 

 temperature was kept at that point at which crystallisation began within 0°02 C 



The general relations observed are as follows : — The proportion of ammonium 

 chloride in the salt increases with that in the solution nearly in direct ratio for a 

 given temperature, the ratio varying from about thrice the proportion of ammonium 

 chloride in the salt that there is in the solution, at the highest temperature, up to 

 nearly six times the proportion at the lowest. The same solution j'ields crystals 

 containing more ferrous chloride at a higher temperature and less at a lower. 

 Solutions containing two or more molecules of ammonium chloride to one mole- 

 cule of ferrous chloride yield well-formed crystals, white and transparent, having 

 sides of one millimetre or more in length. When the salt contains twenty or thirty 

 molecules of ammonium chloride to one molecule of ferrous chloride the crystals are 

 as large as when the proportion of ammonium chloride is smaller. With less than 

 two molecules of ammonium chloride the crystals are apt to be very small and to 

 resemble those of ammonium chloride in form though not in composition. 



In most cases the composition of the crystals can be represented by a formula 

 Fe CI", JiH^NCl where n is integral. Salts have been analysed with closely con- 

 cordant results in which n has the following values: — 7, 11, 12, 13, 15, 16, 17, 

 18, 19, 20, 23. But in some cases, where equal or greater care has been taken that 

 the conditions may not change during crystallisation, the values of n are not 

 integral. Also the double salt is hydrated ; the determination of the amount of 

 water presents greater diiliculties than the rest of the analysis, a temperature of 

 about 200° C. being necessary to expel the whole ; the results thus obtained, which 

 generally agree with the estimation of water by difference, frequently correspond 

 to 2'5 molecules of water for each molecule of ferrous chloride. Perhaps, there- 

 fore, the formulae of the double salts should be multiplied by two, or some larger 

 even number. 



The whole of the results obtained can be grouped together in a Table, in which 

 each salt is represented by the value of n and is assigned a position showing (1) 

 the composition, (2) the temperature, of the solution in which it was formed. 



