302 



Mr. G. F. Rodwell on the Effects of Heat 



cliloride ; and perhaps the same cause as that which lowers the fusing- 

 point lowers also the point of maximum density. 



Fusing-point. — While the fusing-point of iodide of silver has been esti- 

 mated at 450° C, of bromide at 380° C, and of chloride at 350° C, that 

 of the alloys is lower than any of the constituents (except Ko. 5, which is 

 the same as that of the lowest of its constituents, while it contains 74 

 per cent, of the constituent with the highest fusing-point, viz. 450° C). 

 Thus Xo. 1 melts at 330° C, Xo. 2 at 295° C, Xo. 3 at 320° C, Ko. 4 

 at 330° C, and Xo. 5 at 350° C. The most distinctiA^e alloy, Iso. 2, melts 

 at a temperature which is 155° C. below that of the iodide which consti- 

 tutes 41-5 per cent, of the weight of the alloy, 85° C. below that of the 

 bromide which constitutes 33 per cent, of the alloy, and 55° C. below that 

 of the chloride which constitutes 25 per cent, of the alloy. Xow it is well 

 known that in the case of numerous alloys the fusing-point is lower than 

 that of the mean fu sing-points of the components ; further, it is kno-^Ti 

 that a mixture of the fused chlorides of sodium and potassium has a 

 lower fusmg-point than the mean of the constituent salts. Dr. Mat- 

 thiessen says, " It is generally admitted that matter in the solid state 

 exhibits excess of attraction over repulsion, whilst in the liquid state 

 these forces are balanced ; and in the gaseous state repulsion predomi- 

 nates over attraction." Let us assume that similar particles of matter 

 attract each other more powerfully than dissimilar ones. It will then 

 foUow that the attraction subsisting between the particles of a mixture 

 will be sooner overcome by repulsion than in the case of a homogeneous 

 body : hence mixtures should fuse more readily than their constituents. 

 We are at least reminded of the fact that certam perfectly inert bodies, 

 when mixed with substances which decompose at a certain temperature, 

 lower the temperature of decomposition. 



Of the contraction of the alloys between 124° C. and 133° C. — It is a 

 curious fact that until the percentage of iodide of silver in the aUoy be- 

 comes considerable, the chief influence of the iodide seems to be exerted 

 between that narrow range of temperature ; and more than this, that so 

 soon as the contraction is over, the mass undergoes far more rapid ex- 

 pansion than do any of its constituents when heated through the same 

 range of temperature. It is further noticeable that the amount of con- 

 traction in some of the alloys exceeds that of the iodide itself, while we 

 know that the other constituents possess high coefficients of expansion. 

 This is all dependent, without doubt, upon the manner in which the 

 iodide changes its condition ^^ithin the mass of the alloy. Let us take 

 the case of one of the intermediate aUoys, say jN'o. 3 ; in every 100 mole- 

 cules between the temperatures of 124° C. and 133° C. we have 58 

 molecules undergoing somewhat rapid contraction, while 42 are under- 

 going expansion ; at the same time other events are taking place within 

 the mass, heat is disappearing as internal work, and is changing the 

 crystalline into the amorphous iodide, converting an opaque, brittle, 



