xviii REPORT—1852. 
some of the most important elements. But much more than has been already 
performed still remains undone. Such a subject it is believed might be 
highly proper for consideration by the Chemical Section, to whose notice it 
would be introduced by the distinguished chemist, Dr. Andrews, who pre- 
sides over that Section, and than whom no one could be named as more 
competent to estimate the importance of such a revision, or to judge more 
truly of the qualifications that would be required for its execution. 
We are deprived by the illness, I trust only temporary, of our valued asso- 
ciate Prof. James Forbes, of the Report he would have given us of the progress 
of the experiments which he has undertaken at the request of the Association 
to test the Theory of Heat. But this branch of Physics abounds more 
perhaps than any other at the present time in subjects which may be most 
profitably discussed. The theory of Heat has made great advances within 
the last ten years. Mr. Joule has by his experiments confirmed and illustrated 
the views demonstrated about the end of the last century by Davy and Rum- 
ford regarding the nature of heat, which are now beginning to find general 
acceptance. He has determined with much accuracy, the numerical relation 
between quantities of heat and of mechanical work. He has pointed out the 
true principles upon which the mechanical value of any chemical change is 
to be estimated, and by very careful experiments he has arrived at numerical 
expressions for the mechanical equivalents in some of the most important 
cases of chemical action, in galvanic batteries, and in combustion. These 
researches appear to be laying the ground-work for the ultimate formation 
of a Mechanical Theory of Chemistry, by ascertaining experimentally the 
mechanical equivalents expressed in absolute motive force of the thermic, 
electric and magnetic forces. Mathematical developments of the theories of 
heat and electro-dynamics, in accordance with these principles, are given in 
various papers by MM. Helmholz, Rankine, Clausius and Thomson, published 
principally within the last two years. In discussing these subjects the Sec- 
tion will have a great advantage in being presided over by the last-named of 
these gentlemen, a native of Belfast, who at so early an age has attained so 
high a reputation, and who is taking a leading part in the investigations 
to which I have referred. 
In connexion with the subjects of Heat, I would advert to the experiments 
in which Mr. Hopkins is engaged for investigating the possible influence of 
high pressure on the temperature at which substances, in a state of fusion, 
solidify—an inquiry which was shown by Mr. Hopkins, in a report recently 
presented to the British Association, to have an important bearing on the 
questions of the original and present state of the interior of the earth. It is 
well known that the temperature of the earth increases as we descend, and 
it has been calculated that at the rate at which the increase takes place in 
such depths as are accessible to us, the heat at the depth of eighty or a hun- 
dred miles would be such as to fuse most of the materials which form the 
solid crust of the globe. On the hypothesis of original fluidity, and assuming 
