336 M. Edlund on Thermal Phenomena. 



wire contracted while raising a weight. This latter result is quite 

 in accordance with theory. 



M. Edlund experimented on a second steel wire quite like 

 the preceding, on a silver wire 1*14 millim. in diameter, on a 

 German silver wire 1*32 millim. in diameter, on a brass wire 

 0*87 millim. in diameter, on a platinum wire 1*92 millim. in 

 diameter, and on a wire of aluminium-bronze*. All these expe- 

 riments have given results analogous to the foregoing, and which 

 it appears superfluous to repeat in detail. M. Edlund sums up 

 his conclusions in the following general laws : — 



I. If a metal is expanded without the limit of elasticity being 

 exceeded, it undergoes a cooling proportional to the mechanical 

 force by which the dilatation is produced. 



II. If the metal then reverts to its original volume, and effects 

 an external mechanical work equal to that which has been expended 

 in dilating it, it experiences a heating equal to the preceding cool- 

 ing, and therefore proportional to the mechanical force by which the 

 metal was maintained in its state of expansion before contracting. 



III. If, on the contrary, the metal returns to its original volume 

 without producing any external mechanical work, it becomes more 

 heated than in the preceding case, and the difference of the two 

 heatings is proportional to the external mechanical work which in 

 the one case the metal effects at the same time as it contracts. 



IV. It follows from these three principles, that if, without ex- 

 ceeding the limits of elasticity, a metal passes from the volume V 

 to the volume Vj, the accompanying calorific effect does not depend 

 merely on the original and on the final volume, but also on the man- 

 ner in which the total change has been effected. 



M. Edlund remarks that when a metal contracts without effect- 

 ing an external mechanical work equal to that which has been 

 necessary to expand it, its molecules return to their original posi- 

 tions of equilibrium ivithout velocity, because the tension of the 

 wire decreases in the same ratio as the elongation. On the con- 

 trary, if contraction takes place without anyexteimal mechanical 

 work, the motion of the molecules is incessantly accelerated, and 

 they arrive at their positions of equilibrium endowed with velo- 

 cities which may be regarded as identical with the excess of heat 

 disengaged. It might be said that the external mechanical work 

 hinders the production of a certain quantity of heat which would 

 naturally be developed. 



Analogous considerations may be applied to the older experi- 

 ments of Joule on the thermal effects of the expansion of gases. 



* Containing 2'5 per cent, of aluminium, according to an analysis of M . 

 Ullgren. 



