PRESERVATION BY CANNING. 245 



mixed with spices, both for the purpose of adding flavor and of aid- 

 ing in the preservation. Vinegar pickles will not keep indefinitely, for 

 after a time a scum grows over the surface. This is made of micro- 

 organisms which gradually weaken the strength of the vinegar until 

 the final decay of the pickles is only a matter of time, unless precau- 

 tion is taken to prevent the deterioration of the vinegar. 



Spices. Many common household spices are more or less 

 efficient as antiseptics and tend to delay putrefaction. In some 

 kinds of pickles spices are used, and mince meat, sausages, and 

 highly spiced fruit cakes are preserved chiefly by the spices they 

 contain. 



PRESERVATION BY CANNING. 



One of the most important methods of preserving perishable 

 food products was invented a century ago, long before the signifi- 

 cance of bacteria was known and long before the meaning of the 

 process was understood. It was invented by Appert, a Paris con- 

 fectioner in 1810, and consisted first in boiling the material to be 

 preserved and subsequently sealing it hermetically. It was at first 

 supposed that the significance of the sealing was to prevent the access 

 of air, but it is now known that its purpose is simply to prevent the 

 entrance of bacteria; for if these can be kept out, the presence of air 

 does not interfere with the preservation. It is interesting to note 

 that this method of preservation of food products was invented and 

 put to a wide practical use while scientists were disputing and labori- 

 ously experimenting over the problem of spontaneous generation. 

 The experiments by which scientists tried to settle this question con- 

 sisted in exactly the same devices as just mentioned, viz., the heating 

 of various organic materials to a high temperature in order to kill all 

 living organisms and then, after hermetically sealing, watching to 

 see if life developed in the sterilizing mass. While the scientists 

 were disputing as to the results, the method of canning was put into 

 practical use, and every can of preserved fruit was evidence against 

 spontaneous generation. 



The general method adopted in canning is well known. Micro- 



