RELATION TO ENVIRONMENT 151 



which either the water in the solution or a part of the formalde- 

 hyde reacts, producing heat and the evolution of gas, which 

 reaction liberates the formaldehyde gas. The substance most 

 frequently used is potassium permanganate. When potassium 

 permanganate and formaldehyde are brought together in the 

 right proportions, there is a violent reaction and the formalde- 

 hyde gas is rapidly given off, together with a considerable 

 amount of moisture. A larger amount of formaldehyde is 

 needed to disinfect a given space by this method than those 

 considered above because of the fact that part of it is wasted 

 in the chemical combination, so that this method is somewhat 

 more expensive than others. But, due to the fact that it can 

 be used without any special apparatus, and that practically 

 the materials are universally available and that its efficiency 

 is very high, it is at the present time the method par excellence. 



Formaldehyde gas is exceedingly efficient in its germicidal 

 action on bacteria. It has a more or less selective action and 

 does not affect, or at least readily, many other kinds of cells 

 such as those that compose the tissues of higher plants and 

 animals. It does not kill plants or ordinary animals, such 

 as insects, rats, and mice, in the same concentration required 

 for the destruction of bacteria. This selective action is dis- 

 advantageous as well as desirable. On account of this fact 

 sulphur is still used where it is necessary to kill other forms 

 of life than bacteria. 



Formaldehyde gas has practically the same specific gravity 

 as air. It therefore readily diffuses throughout the air space. 



