PHYSICO-CHEMICAL PROCESSES 149 



consumed. The oxygen then unites with the hydrogen 

 and leaves the carbon in fine particles of soot or smoke, 

 which float away with the draught or are deposited upon the 

 surface of the boiler. Moreover, when the air is not suffi- 

 ciently hot, partial combustion again results, changes the 

 hydrogen to water -vapor, and sets the carbon free as soot or 

 smoke. If the gases become chilled, and pass off as a whole 

 unburned, they thus carry away, not only their own heat 

 of combustion, but also the heat which has been absorbed 

 for their liberation. Smoke is therefore the sign of the 

 imperfect combustion of hydrocarbons. 



165. Chemical Bacteria. Animal grease is not suitable 

 as a lubricant because it soon becomes " rancid," that is, 

 it gives off a disagreeable odor and forms acids. Careful 

 experiments show also that the changes which take place 

 in grease and other organic substances when exposed to 

 warm, moist air are caused by small living plants or organ- 

 isms. When these minute organisms alight upon certain 

 vegetable and animal substances, they grow vigorously, 

 and live on the material. As the result of their action, a 

 chemical change takes place. In the case of starch or sugar 

 this change is called fermentation; in the case of fat, ran- 

 cidity; and in the case of proteids (compounds of nitrogen, 

 carbon, oxygen, and hydrogen), putrefaction. These living 

 organisms are called microbes, germs, and bacteria. 



All the changes that take place in milk, such as souring, 

 becoming tainted, etc., are due to bacteria. Cream, as it 

 is obtained from milk, contains bacteria in large quantities, 

 and as these organisms grow they produce the ripening effect 

 which gives flavor to the butter. Certain species of bacteria 

 carry disease and produce undesirable effects upon the 



