B72 DOLBEAR. (VoL. II. 
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motion that constitute what is called heat, and are therefore 
measurable in heat units. As a heat unit can do a definite 
amount of mechanical work, namely, raise 426 kilograms a metre 
high, this latter quantity is called the mechanical equivalent of 
heat. Chemical work is therefore expressible in mechanical 
units. For example, when a kilogram of hydrogen combines 
with oxygen to form water, there are given out in the operation 
34,000 heat units, capable of doing 426 X 34,000 = 14,484,000 
kilogrammetres of work. A kilogram of coal when burnt gives 
out 8080 such units, and the elements thus combined can only 
be separated again by the expenditure of as much energy upon 
them as they gave out when combining. This department 
of chemistry is known as Thermo-Chemistry. The relation 
between heat and work, as explained above, is known as the 
first law of Thermodynamics. 
Again, from the second law of Thermodynamics it may be 
shown that the total heat energy of a body, or its ability to do 
work, is proportional to its absolute temperature. Hence at 
absolute zero no chemical work could be done; that is, chemism 
does not exist, and no work would be spent against cohesion to 
separate the elements in a molecule. From this it may be 
inferred that chemism is a derived property, and is the result 
of heat; and also that Chemistry itself is a department of 
Thermodynamics. 
In order to understand more definitely how heat can bring 
about such phenomena as selective combinations and the organ- 
ization of atoms into molecules and crystalline forms, it becomes 
necessary to know more precisely what the nature of heat 
motions is that enables it to produce such mechanical effects. 
If one turns to the text-books for a definition of heat, he will 
quite likely be perplexed by the different statements to be found 
in them. One may say that heat is energy; another, that it 
consists of ether waves; a third, that it is a mode of motion; 
and still others treat it in all three senses. The trouble comes 
from the failure to distinguish between heat and some of the 
effects of heat. Really they all agree that heat is a kind of 
atomic and molecular vibration, as distinguished from oscillatory, 
rotary, or free-path motions, and that is the meaning of it as 
applied here. It is a necessary change of form of the atoms 
or molecules, and will be easily understood by considering the 
diagram, Fig. 1. 
