INTRODUCTION. 19 



tion of acetic acid; of those which contain nitrogen, 

 they are mostly alkaline, from the formation of ammonia 

 and other bases. 



Many organic substances, especially such as contain 

 nitrogen, are decomposed when exposed to the influ- 

 ence of air and water at ordinary temperatures, their 

 elements being rearranged during the process to form 

 simpler substances. r fhis kind of decomposition is 

 called putrefaction. Putrefaction occurs only under 

 certain conditions. It can only take place in the pre- 

 sence of water, and access of atmospheric air is neces- 

 sary to its commencement. Once begun, however, it 

 continues without access of air. Everywhere in the 

 air are present microscopical germs of vegetable and 

 animal organisms. When these fall on a soil favor- 

 able to their growth they are developed quickly, they 

 multiply with great rapidity, and in consequence of 

 the vital process and the dying off of these organisms, 

 that species of decomposition of organic compounds 

 takes place which is called putrefaction. The air loses 

 its power to start the process of putrefaction when pre- 

 viously passed through a strongly heated tube, or a 

 dense cotton stopper, or even only through a tube 

 which has a large number of curves, as by these means 

 the germs, which are present in the atmosphere, are 

 either destroyed or held back. Further, putrefaction 

 occurs only within certain limits of temperature, most 

 readily between 20 and 30. Below and above 

 100 it does not take place. 



If the oxygen of the air takes part in the decompo- 

 sition, and thus a simultaneous oxidation takes place, 

 the decomposition is called decay. The last products 

 of decaying organic substances are water, carbonic 

 acid, and ammonia. 



A phenomenon very similar to putrefaction is fer- 

 mentation. This will be treated of more in detail in 

 connection with alcohol. 



Organic compounds can be changed in a variety of 

 ways under the influence of many inorganic bodies. 



Free oxygen acts on but a very few organic bodies 

 at the ordinary temperature ; it acts, however, more 



