CATALYSIS AND CATALYSTS 199 



siderable amount of heat, which, if allowed to accumulate, so as 

 to raise too high the temperature of the tubes containing the 

 contact substance, will partly undo the result of the combination, 

 and the sulphur trioxide is destroyed. Another point to attend 

 to is the necessity for providing a considerable excess of oxygen 

 in the form of atmospheric air, the nitrogen of which takes no 

 part in the change. The platinum is used in the form of a 

 deposit of fine particles on asbestos fibre, which is easily produced 

 by soaking the asbestos in a solution of platinic chloride, drying 

 it and then exposing to a low red heat by which the chloride is 

 completely decomposed. Since platinum has become so costly 

 as it now is, many attempts have been made to replace it by 

 other substances, and many patents have been taken out. It 

 appears, so far as can be ascertained, that ferric oxide (red oxide 

 of iron) has met with some success, but has not served to replace 

 platinum. 



In connection with the use of platinum a discovery was made 

 in the earliest days of this process which for a time checked its 

 development and even threatened failure. The fact came 'out 

 that minute quantities of certain substances have the property 

 of " poisoning " the catalyst, so that its activity pretty rapidly 

 declines, and it becomes " dead." Fortunately this was traced 

 to the impurities which accompany the sulphur dioxide produced 

 by roasting iron pyrites. Of these the most important is arsenic. 

 By cooling and spraying with water the gases brought from the 

 pyrites ovens these impurities can be removed and the gas, 

 cleared of mist, can be safely delivered into the series of pipes 

 charged with the platinised asbestos. Many other practical 

 points require attention to secure success in the operation. 

 Thus it is found that at a temperature of 400 to 430, 98 

 to 99 per cent of the sulphur dioxide is converted into the 

 trioxide, while a further rise of temperature reduces the 

 yield. It is therefore necessary to provide the means of 

 cooling by a draught of air when the temperature tends to rise 

 too high. 



The Claus kiln affords another example of a catalytic process 

 which has been turned to account for industrial purposes. In 

 the production of soda, that is sodium carbonate, from common 

 salt by the Leblanc process which has been in operation for more 

 than a century, the accumulation of impure calcium sulphide in 

 the form of alkali-maker's waste was for generations a 



