1868. ] The Past and Present of Chemistry. 27 
pounds are regarded as being composed either of different materials, 
or of the same materials in different proportions. ‘Tin, silver, 
mercury, and sulphur are regarded as fundamentally dissimilar 
substances; but in wood, in alcohol, in acetic acid, and in many 
other bodies, the same substances, vzz. carbon, hydrogen, and oxy- 
gen, are contained in different proportions. 
Until about the year 1830 it was believed that the chemical 
differences of bodies depended solely upon the causes just enu- 
merated. It was supposed that the same element could only pre- 
sent itself in one form, endowed with one invariable set of properties, 
and that from the combination of the same elements in the same 
proportions, only one and the same substance could possibly result. 
But at this period facts became known which rendered this opinion 
untenable. At first these facts were few, and were regarded as 
exceptions to an essentially valid law, but their number rapidly 
increased, so that what had previously been considered as an inyvari- 
able law was now shown to have owed its invariability to limited 
knowledge, and it became obviously insufficient to explain the 
chemical differences of bodies. Vinegar and sugar are, even from a 
purely chemical point of view, widely different substances, but they 
are composed of precisely the same elements, viz. carbon, hydrogen, 
and oxygen, combined in exactly the same proportions. An extin- 
guished spirit-lamp, the wick of which is still red, emits an intoler- 
ably pungent smell ; the very volatile liquid which is here produced 
and which emits this suffocating odour, has the somewhat uncouth 
name, aldehyde. Acetic ether, which has a delicious and refreshing 
smell, possesses entirely different chemical properties. Again, butyric 
acid, which is contained in rancid butter, and which has a most dis- 
gusting odour, is a body differig widely from aldehyde and acetic 
ether. Nevertheless, these three substances, which differ so mark- 
edly, both as regards chemical and physical properties, consist of the 
very same elements, viz. carbon, hydrogen, and oxygen united in 
precisely the same proportions. 
In the case also of some elementary bodies such a variation of 
properties has been observed, that it would be almost impossible 
a priori to imagine that it is one and the same substance which 
appears in such various forms. ‘The elementary body, phosphorus, 
which has been known for nearly 200 years as a soft, yellow sub- 
stance, fusing easily, exceedingly inflammable, and undergoing rapid 
change when exposed to the air, appears also in the form of a red, 
brittle material, capable of enduring a high degree of heat without 
inflaming or undergoing any alteration, even when simultaneously 
exposed to the air. Again, the element oxygen, contained in large 
quantities in the air, which has no smell, acts upon certain sub- 
stances only at a high temperature, and may remain for a long time 
in contact with moist silver without producing any change in the 
