1863.] 547 



When a portion of Witherite is enclosed in a tube with a strong 

 solution of protochloride of iron, there is a slow decomposition into 

 chloride of barium, which is dissolved, and carbonate of iron, which 

 remains firmly attached to the Witherite, and would ultimately give 

 rise to an excellent pseudomorph. The best conclusion at which I 

 have been able to arrive is, that there is in this change an increase in 

 volume equal to about 10*7 per cent, of the Witherite altered, so 

 that, under pressure, mechanical force must be overcome. In an 

 experiment where everything went on in a very satisfactory manner, 

 the pressure was maintained for three months at from 80 to 100 

 atmospheres, and for one month was under 80 atmospheres, so that, 

 on an average, it was about 80 atmospheres ; and I found that the 

 amount of chemical change was 21*7 per cent, less than when, all 

 other circumstances having been the same, there had been no pres- 

 sure ; thus clearly showing that pressure had, as it were, diminished 

 the force of chemical affinity. If then one cubic inch had been altered 

 under this pressure, it would have overcome a mechanical force equal 

 to that required to raise 1200 Ibs. through the space of -107 inch, 

 which is equivalent to raising twenty- one times its own weight to the 

 height of 1 metre; and under the same circumstances T278 cubic 

 inch would have been altered when no such mechanical force had 

 to be overcome. Supposing then that in both cases the total energy 

 at work was the same, but in one was altogether expended in pro- 

 ducing a chemical result, and in the other in producing partly a 

 chemical and partly a mechanical effect, we may say that the force 

 which gives rise to the purely chemical change, taking place at a 

 particular rate, is equal to that which gives rise to this chemical 

 effect, taking place at '783 of that rate, and to a mechanical 

 effect equal to the force required to raise in the same space 

 of time 34-87 times the weight of the Witherite altered to the 

 height of 1 metre. Supposing also that the power of chemical 

 force varies as the rate at which it gives rise to a chemical change, in 

 the same manner as the power of a mechanical force varies as the 

 velocity of motion imparted by it, we may perhaps conclude that this 

 mechanical force is equal to '217 of the chemical force, and that the 

 whole energy of the chemical action under the conditions of the ex- 

 periment was equal to the mechanical power required to raise in the 

 same period of time 160 times the weight of the Witherite altered to 



