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- 
