124 SCIENCE PROGRESS 



Hatschek has produced forms closely resembling those of 

 medusae and other aquatic beings. 



Another suggestive property of colloids is their power of 

 greatly accelerating chemical changes that, otherwise, would 

 occur to an inappreciable extent. The " catalytic " power of 

 finely divided platinum is utilised in a popular type of gas 

 lighter. When brought within the stream of gas, energetic 

 combination with the oxygen of the air is induced, and the heat 

 evolved is sufficient to kindle the gas. A sheet of the metal is 

 able to bring about the gradual combination of oxygen and 

 hydrogen. The more intense effect of the " platinum black " 

 is due to the increased surface. The actual part played by the 

 platinum in such reactions has been much discussed. It has 

 been suggested that a hydride of platinum is formed, that the 

 gases are dissolved in the metal, or that they are adsorbed at 

 the surface. It is becoming increasingly evident, however, from 

 consideration of the electrical structure of matter, that the 

 attractive forces exerted by molecules are the same as those 

 causing chemical combination. Whether the phenomenon be 

 regarded as physical or chemical, it is probable, therefore, that 

 the unlike gaseous atoms are brought so close together under 

 great pressure, by the forces exerted at the metallic surface, that 

 the comparatively weak fields of force between the like atoms 

 composing the gaseous molecules are disturbed, the greater 

 attraction between the unlike atoms comes into play, and a 

 compound of oxygen and hydrogen is formed. 



In such a chemical change as we have described the 

 '' catalyst " is recovered intact after the action, and a very 

 small amount of catalyst is able to transform a large amount of 

 material. Catalytic reactions are of great and increasing im- 

 portance in chemical technology. Spongy platinum is used to 

 determine the combination of sulphur dioxide with oxygen in 

 the " Contact " process of sulphuric acid manufacture. Finely 

 divided nickel may occasion the change of acetylene into 

 various kinds of petroleum according to the temperature. 

 The same metal, and others, are used in a process, known as the 

 hydrogenation of oils, whereby liquid unsaturated hydrocarbons 

 are converted into solid edible fats for making margarine. The 

 bulk of the world's production of formalin is made from wood- 

 spirit and air through the agency of a metal catalyst. The 

 catalytic synthesis of ammonia, from the nitrogen of the air 

 and hydrogen, made it possible for Germany to continue the 

 war when foreign sources of combined nitrogen had been 

 cut off. 



Apart from their commercial importance, however, catalytic 

 changes are of the greatest interest in consequence of their 

 occurrence in the living organism. The cellular reactions 



