VELOCITY OF REACTIONS 21 



constituents of air, which gave the arterial blood 

 its brilliant red colour. Van Helmont described 

 the carbonic acid, gas sylvestre, which is evolved 

 in the process of fermentation of wine or beer. 

 In his famous work, Experiments and Observations 

 on different Kinds of Air, Priestley described 

 the action of plants on air deteriorated through the 

 respiration of animals. He showed that the green 

 parts of the plants in sunlight decompose carbonic 

 acid and give off oxygen to the air. In this 

 way the plants and the animals counteract each 

 other and help to keep the composition of the air 

 unchanged. This problem attracted, by its great 

 practical importance, the chief interest of bio- 

 chemists for a long time. The most important 

 investigations in this chapter we owe to Senebier 

 and Ingenhouss in the eighteenth century, to de 

 Saussure, Dumas, Liebig, Daubeny, Draper, 

 Sachs, Baeyer, Pfeffer, Engelmann, and Prings- 

 heim in the nineteenth century. Baeyer ex- 

 pounded the prevailing theory that the plant 

 products from carbonic acid and water are oxygen 

 and formaldehyde, which through polymerisation 

 gives the different carbohydrates, such as sugar 

 or starch or even cellulose. In recent time Daniel 

 Berthelot, Stoklasa, and others have succeeded 

 in carrying this process through without the help 

 of green plants by means of ultra-violet light. 



In an analogous manner Duclaux imitated the 

 chief fermentation process, by which alcohol is pro- 

 duced from sugar by the agency of yeast-cells, by 



