286 



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



contraction became more rapid; and between 142° and 145°-5 C. very 

 rapid contraction took place, and the temperature was maintained as 

 steady as possible. A slight fall o£ the temperature below 142° C. 

 caused the needle to stop, and the contraction was resumed as soon as 

 the temperature again rose above that point. When the temperature 

 was kept steadily between 142° and 145°-5 C, the contraction after 

 a while ceased at the latter temperature ; and on raising the temperature 

 the mass now expanded slowly up to 300° C, the limit of the apparatus, 

 as beyond this the parafhne ignited. In cooling these effects were 

 exactly reversed : the index moved from 180° of arc to zero between 300° 

 and 142° C, that is, the mass contracted like an ordinary solid ; then at 

 142° a considerable expansion, occurred, after which the mass slowly ex- 

 panded as the temperature fell. 



rizeau, " by a method depending oii the accuracy with which extremely 

 minute movements can be appreciated, by observing the changes they 

 produce in a system of Newton's rings," determined the coefficient of 

 cubical contraction of iodide of silver between -f- 70° C. and — 10° C, 

 and found it to be -00000417. The determinations made with the 

 expansion-apparatus were higher than this ; but as M .Fizeau's method is 

 undoubtedly one of great accuracy, and as, moreover, the determinations 

 of the expansion of silver, copper, lead, zinc, and of the chloride and 

 bromide of silver made by means of the expansion-apparatus agree very 

 closely with those made by M. Fizeau by his optical method, I have pre- 

 ferred to adopt his coefficient for temperatures between — 10° C. and 

 + 70° C. Between 70° C. and 142° C, at which temperature the con- 

 siderable expansion occurs, the coefficient was found to be '00001749 ; 

 the determination is apt to be vitiated by the sudden increase in the co- 

 efficient as the temperature of rapid contraction is approached. Between 

 142° C. and 145°-5 C. the mass contracts to the extent of -004500 ; 

 probably this contraction occurs at one precise temperature within even a 

 less range than 3°'5 C. ; but it is practically impossible to ensure an abso- 

 lutely uniform temperature in a bar of iodide in a bath of paraffine 

 under the conditions of the experiment. Between 145°-5 C. and 300° C. 

 the coefficient was found to be -00002844. The iodide fuses at a tem- 

 perature which I estimate at 450° C. By fusion in the platinum cone, 

 and the application of the method described above, the volume in the 

 liquid state at 450° C. was found to exceed the volume at 450° C. in the 

 solid state to the extent of -046331 on volume at 450° C. in the solid 

 state = 1-008659. On raising the temperature of the platinum cone 

 and its contents to a temperature estimated at 750° C. (a cherry-red 

 heat visible in broad daylight), the molten iodide did not appear to 

 expand more than the platinum ; the coefficient between 450° C. and 

 750° C. has been hence taken as that of platinum as an approxi- 

 mation. As no method could be devised for determining the coefficient 

 of expansion between 300° C. and 450° C, the same coefficient as that 



