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varied phenomena presented by the elements that evade the 
direct tests of weight and measure — such as light, electricity, 
and magnetism, and more especially that called heat or caloric. 
With respect to the nature of the latter, directly conflicting 
views have been taken by philosophers of eminence, both 
ancient and modern, in expounding the causes of calorific 
phenomena, — on the one hand denying and on the other 
aflirmiug the identity or reality of heat as a natural element. 
Among the ancient asserters of the nonentity doctrine, we 
find the names of Boyle and his followers; and in modern 
times those of Count R-umford, Sir Humphry Davy, Dr. 
Joule, and many others of less eminence : and in support of 
the material nature of heat, we have the names of Boerhaave 
and his school, and in later times those of Dr. Black, James 
Watt, Sir James Leslie, Dr. Hulton, Dr. William Henry, and 
Dr. Dalton, with others of note. Without here discussing 
the diverse grounds of difference on the questions at issue 
(having elsewhere dwelt at length on their respective bear- 
ings) I proceed to explain those on which the materiality 
of heat appears to rest, and whereon the entire range of 
phenomena connected with the imponderable elements may 
admit of rational solutions. 
I assume the existence of four distinct states or conditions 
of heat, and which are thus defined, viz. — (1) elemental or 
neutral heat, (2) latent or combined heat, (3) sensible or 
expanding heat, and (4) radiating heat. Of the two last 
forms little need be said beyond what is directly seen in 
their alternate changes into and out of the other forms of 
heat (explained further on). The existence of a pervading 
medium in space has been admitted by the most eminent 
physicists; but no explanations are given of its nature, 
except that of its elasticity, nor yet of what are the rela- 
tions of this medium to those of light, heat, electricity, and 
magnetism. It is treated as a pervading “ether,” or “lu- 
minous ether,” and its resistance to the transit of light 
