The Romance of Modern Chemistry

SMALL CAUSES; GREAT EFFECTS suffocating gas which is produced when sulphur is burned —to combine with more oxygen. This will not occur spontaneously when the gases are merely mixed, even at a high temperature. They must be brought into con- tact with some third substance which plays the same part as water does in the combination of hydrogen and oxygen. ’ In the case of sulphur dioxide and oxygen the third party, which acts in some subtle way as mediator between the other two, is platinum in a finely divided condi- tion. This metal has quite a reputation for accelerating chemical actions in which it is not directly involved; it is a sort of chemical busybody. There is a well-known ex- periment which illustrates this characteristic of platinum very clearly indeed. A roll of platinum foil is suspended in the flame of a Bunsen burner until it is red hot; the gas is then turned off, and immediately turned on again, but not lighted. The observer sees that the foil, which had begun to cool down whenever the gas was turned off*, begins to glow afresh, although there is no visible flame; it remains in this condition so long as the mixture of air and gas from the burner is allowed to flow over it. What has happened is that the platinum induces the slow combustion of the gas, and it is the heat given out in this process which keeps the metal visibly hot. The platinum itself is not affected, so that we have here an excellent example of catalytic action. A similar part is played by the finely divided platinum in the modern method of making sulphuric acid—the “ contact ” process, as it is appropriately called. Under the persuasive influence of the catalytic agent at a tem- perature of about 500° Fahrenheit, sulphur dioxide and oxygen readily unite to form another compound named