VEGETABLE GARDENING 57 
food but also form certain products which are poi- 
sonous. We call these ptomaines. Contrary to popu- 
lar conception, ptomaines do not necessarily come from 
tin cans. They are produced by the action of bacteria 
on food and are probably the secretions of the bacteria. 
All of this information about bacteria has been ac- 
quired by man very gradually. He soon learned to dry 
foods and by removing the water, to starve the bacteria, 
yeasts and molds. The Indians used this method in 
preserving buffalo and deer meat. Salting and pickling 
are effective methods of food preservation also. The 
‘ use of drying has been developed to such an extent 
that now milk is evaporated and kept for some time 
this way. 
Keeping foods cool. The spring house, cool cellar 
or family ice box is a short time cold storage plant 
where preservation is a matter of days. In the city 
cold storage warehouse food products can be kept for 
months. 
The following rules should be observed in the house- 
hold in preserving foods by this method: 
1. Cool the food as quickly as possible. This should 
be done before covering and setting aside for keeping. 
2. Use every possible device for avoiding moisture, 
8. Use food quickly after taking it from its place of 
storing, for such food. when warmed decays rapidly. 
Canning a safe method. Preservatives are often 
used by commercial firms and even in households to 
preserve food. : The use of such chemicals is dangerous 
and unnecessary. Food products may be canned and 
kept in perfect condition without their use. 
Canning is comparatively a new method of food 
preservation. During the Napoleonic Wars, the’ great 
Napoleon, originator of that famous expression “An 
