212 CHEMICAL DISCOVERY AND INVENTION 



fuel in which the liquid is first burnt in a separate space under 

 the boiler, and the imperfectly burnt products are carried with 

 the requisite proportion of air through the tubes containing the 

 granular contact substance. 



In the third arrangement the granular material is placed in 

 trays beneath the boiler. 



The new method admits of the employment of almost any 

 form of combustible gas, such as waste gases from the blast 

 furnace or coke-oven, producer-gas of any kind, as well as water- 

 gas and coal-gas. A high efficiency has been obtained up to 

 92-94 per cent in the most favourable cases. 



Any attempt to explain the process of surface combustion 

 involves many considerations for which at present we are not 

 fully prepared. Some important facts, however, have already 

 been revealed. Thus, as already mentioned in the chapter on 

 electrons, it has been discovered that many surfaces when heated 

 emit these small bodies, and this emission appears to be not 

 necessarily connected with combustion in the ordinary sense, 

 though at high temperatures a greatly increased emission of 

 such particles with great velocity may have a good deal to do 

 with the phenomena. The fact that the catalytic surface 

 becomes negatively charged during this kind of combustion is 

 specially significant, and possibly the formation of layers of 

 electrically charged gas may be the seat of a greatly increased 

 chemical activity. 



The chapter began on the subject of catalysis and catalysts, 

 but though it cannot be extended further the subject is by no 

 means exhausted. Everywhere through the literature of chemis- 

 try, old or new, examples occur of processes which must be 

 regarded as operating under the influence of these mysterious 

 agencies. Hence to be exhaustive the whole known range of 

 chemical changes would have to be reviewed. 



.Among processes long recognised as catalytic is the Deacon 

 process for obtaining chlorine. This depends on the interaction 

 of hydrogen chloride gas with the oxygen of air in the presence 

 of cupric chloride. A temperature somewhat over 400 C. is 

 required. The process was introduced more than forty years ago, 

 and will be found described in all the principal manuals of 

 chemistry. 



There are many other cases in which a small quantity of a 

 substance is sufficient to initiate or promote a chemical change 



