Proceedings. 163 



finds himself obliged to select as his most reliable ex- 

 periment, one which gives a value at all different from that 

 of ' the great Manchester Physicist.' 



"Hirn's work in thermo-dynamics was not limited to the 

 determination of the mechanical equivalent, but extended 

 to the theory of heat engines, and he was, I believe, the first 

 to remark that regenerators (or heat economisers) are 

 without useful effect upon an efficient air engine. He 

 does not deny that they may be economically useful 

 upon any particular engine, but points out that the fact 

 of their producing an economy proves the existence of 

 a grave defect in the engine ; and that they can properly 

 be regarded only as a means of raising the efficiency of 

 a defective engine. A very important work of Hirn's was 

 upon superheated vapours, which he undertook in order 

 to elucidate the economical working of engines, and 

 to supply an experimental basis of the Second Law of 

 Thermo-dynamics : and he was the first to notice an ab- 

 normal action of ether vapour. Generally, if at the com - 

 mencement of an operation the vapour is dry and saturated, 

 there is a condensation of the vapour upon expansion, and 

 it becomes superheated on compression, i.e., provided there 

 is no loss or gain of heat ; but in the case of ether he found 

 the action to be reversed. This curious result was ex- 

 plained theoretically by Rankine, and has been experi- 

 mentally verified by M. Cazin, who has shewn that there 

 is a point of inversion for all vapours — that is, a temperature 

 above and below which they act differently. The point of 

 inversion for ether is about -11 5 , while for the vapour of 

 water it is about 520 . The results of Hirn's work in this 

 direction are expressed in what is called Hirn's Law — That 

 the internal work of a superheated vapour is proportional 

 to /^between the point of condensation and that at which 

 it possesses the property of a permanent gas. So that if a 

 gas neither does external work nor gains nor loses heat, 



