52 PRESERVATION OF FOOD BY REFRIGERATION ; 
lately scientific opinion has unanimously decided that heat is not a 
kind of matter, but a kind of energy.’’—Stewart, op. cit. 
“The condition of heat is a condition of energy; that is, of 
capacity to effect changes. 
“The condition of heat, considered as a kind of energy, is 
capable of being indirectly measured, so as to be expressed as a 
quantity, by means of one or other of the directly measurable 
effects which it produces. When the condition of heat is thus 
expressed as a quantity, it is found to be subject, like other forms 
of energy, toa law of conservaéion ; that is, if in any system of bodies 
no heat is expended or produced through changes other than 
changes of temperature, then the total quantity of heat in the 
system cannot be changed by the mutual actions of the bodies, as 
what one body loses another gains, and if there are changes other 
than changes of temperature, then if by those changes the total 
heat of the system is changed in amount, that change is compen- 
sated exactly by an opposite change in some other form of 
energy.’ —Rankine, op. cit. 
“The heat of a body is caused by an extremely rapid oscillat- 
ing or vibrating motion of its molecules, and the hottest bodies 
are those in which the vibrations have the greatest velocity and 
the greatest amplitude. At any given time the whole of the mole- 
cules of a body possess a sum of ws viva, which is the heat they 
contain. To increase their temperature is to increase this ws viva ; 
to lower their temperature is to decrease this ws vzva. Hence on 
this view heat is not a substance, but a condition of matter, and a 
condition which can be transferred from one body to another. 
When a heated body is placed in contact with a cooler one, the 
former cedes more molecular motion that it receives, but the loss 
of the former is the gain of the latter. In solids the molecules 
have a kind of vibrating motion about a certain fixed position. 
This motion is probably very complex; the constituents of the 
molecule may oscillate about each other besides the oscillation of 
the molecule as a whole, and this latter may be a to-and-fro 
motion, or it may be a rotatory motion about the centre. In cases 
then in which external forces, such as violent shocks, act upon a 
body, the molecules may permanently acquire fresh positions. In 
the /¢guzd state the molecules have no fixed positions. They can 
rotate about their centre of gravity, and the centre of gravity itself 
may move. But the repellant action of the motion compared with 
the mutual attraction of the molecules is not sufficient to separate 
the molecules from each other. A molecule no longer adheres to 
particular adjacent ones, but it does not spontaneously leave them 
except to come into the same relation to fresh ones as to its pre- 
