276 
OUR KNOWLEDGE OP 
bility of matter was established^ and the present system of 
classifying bodies according as their constituent parts were all 
alike, or of more than one kind, was also on a firm footing. 
Chemical combination w^as, for a time, regarded as the mere 
union of two or more unlike forms of matter. 
It has, however, been shewn since, that just as the phlogis-. 
tians were wrong in disregarding the weights concerned in 
chemical action, so Lavoisier and his immediate successors (except 
Eichter) fell into the mistake of overlooking the importance of 
the heat which is evolved. Modern chemists have investigated 
this latter point in a vast number of cases, and it appears 
that 'when chemical combination takes place there is always 
heat evolved, and that the amount of heat evolved in the 
combination of the same bodies is constant if the w^eights 
concerned are constant. Thus one ounce of hydrogen, when 
burnt with oxygen^ always produces enough heat to raise 
the temperature of 34034 ounces of water V G, Further, 
it is found that if the water he raised to a white heat, it 
will be reconverted into hydrogen and oxygen ; and these will 
recombine to form water, with evolution of heat as before, 
whence we see that in the decomposition of the water heat was 
taken up, as it had been previously evolved in its formation. 
Thus it seems not perfectly correct to say that water is 
composed of hydrogen and oxygen, in the sense that hydrogen 
and oxygen are actually in the w^ater, for in combiniug a 
certain amount of heat is evolved, and in order to get these 
bodies once more in the form in which they have the properties 
proper to oxygen and hydrogen, it is necessary to supply the water 
with as much heat as was evolved at its formation. In saying 
this, I do not wish to be understood to state that the matter i®f 
hydrogen is not different to the matter of oxygen, and that in 
water these two matters may not both be present ; that may or 
may not be so, but certainly there is an evolution of energy in 
