This high-sounding term means that substances have a power of uniting together that can be better explained by an experiment. Allow a few drops of water to fall on a perfectly clean piece of iron. In a short time a reddish-brown substance will appear on the iron that in ordinary language is called rust. What does this mean? Water is a compound substance composed of oxygen and hydrogen, but when brought into contact with iron the oxygen prefers to unite with the iron and sets the hydrogen free. Hence, would the chemist say, oxygen has a “stronger affinity” for iron than for hydrogen. In this case the rust is composed of rust, a combination of iron and oxygen called oxide of iron. What has taken place may be shown by the following, which will be easily understood:

So all that the chemical combination in the above means is that the iron has taken the place of the hydrogen in the water used for the experiment. If weighed it would be found as always, that the water and the iron weighed precisely the same as the oxide of iron and the hydrogen.

It is to this same principle of chemical affinity that the curious experiments of magic writing with sympathetic inks are possible.