C. The iron was not tested for specific capacity for mag- 
netism; yet this is a most important point which is even 
now hut little appreciated. It is found practically that, if 
two electro-magnets be made from the very same piece of 
iron, most carefully prepared, with the very same length of 
the same wire, without the slightest assignable cause, one 
will sometimes have three times the power of the other. 
Hence I conclude that the maximum energy capable of being 
evolved by a grain of zinc must be very much greater than 
that assigned to it by Dr. Joule. 
7. Dr. Hopkinson’s argument, in his paper lately read to 
this Society, virtually amounted to this — that a well con- 
structed magnetic engine will get no more duty from a 
jjrain of zinc than an ill-constructed one; and consequently, 
I presume, that magnets might be weakened to any extent, 
and removed to ever so great a distance from one another, 
without necessarily affecting the efficiency of the engine. 
8. Dr. Hopkinson has in his criticism strangely substi- 
tuted (a — b) for ( b .) In Joule and Scoresby’s paper, the 
consumption of zinc is expressed not by (a — b) but by (6); 
and consequently the duty of a grain of zinc not by 
W W 
- — - but by— ; and when the magnets are stronger and 
approach nearer to each other, even if W be not increased, 
( b ) is diminished. 
9. My argument was this, that since the accepted theory 
of the mechanical equivalence of heat is that production of 
energy absorbs, and destruction of energy produces, a defi- 
nite amount of heat, if we find cases, as those of elastic 
wires, and water below its maximum density, in which de- 
struction of energy produces cold, not heat, then the doc- 
trine of the mechanical equivalence of heat cannot be true ; 
we might with equal justice call it a mechanical equivalence 
of cold. It is no reply to say that such facts are simple 
deductions from the haws of thermodynamics. This would 
