12 



to interfere in another with its nutritive properties. The food seem less digestible and 

 its " livingness" seem in someway destroyed. The reasons for these subtle changes 

 have not vet been clearly traced.; that they occur is unquestionable. 



We must now pass on to a review of the methods employed. 



(c.) KEEP FOOD PROTECTED FROM THE ATTACKS OF GERMS, YKASTS, AND MOULDS. 



Of these there are several, most of them being quite familar to us. 



(1.) The Exclusion of Air. As air is necessary to the life of most of these micro- 

 organisms, they cannot develop in or attack food from which all air is excluded. 



A favourite domestic application of this method of food-preservation is to cover the 

 surface of potted meat or fish, for example, with a layer of lard or other fat, which is 

 impervious to air. Oil or paraffin-wax serve the same purpose. 



Commercially, oil is also employed to preserve fish, such as sardines, herrings, or 

 anchovies ; and in Italy a few drops of oil seal the necks of wine-flasks, instead of 

 corks. 



The exclusion of air is, of course, one secret of success in jam-making or preserving 

 fruit or vegetables, combined with a very high temperature, which destroys micro- 

 organisms if present. We all know that if a jar is not brimful of boiling fruit and 

 syrup, so that even the tiniest space is left for air to occupy, decay invariably follows, 

 usually in the form of moulds. 



The enormous quantity of 



Canned Food* 



on the market illustrates the wide utilization of this method of food-preservation by air- 

 exclusion, combined with previous sterilization. Ill-effects rarely follow its consump 

 tion ; nevertheless, every one who uses canned foods should never omit to practise the 

 following precaution : 



(a.) Always turn out the contents of any tin into a glass, china, or earthenware 

 dish immediately the tin is opened. This precaution is quite as necessary in the case of 

 fish preserved in oil, though it is usually neglected. 



This precaution is necessary, because the exact extent to which the natural acids 

 of food corrode the inner surface of tins is not yet known ; but injurious metallic salts 

 are occasionally found, and such chemical action is more liable to occur when the tin is 

 opened and the contents become exposed to the external air than when it is hermetic- 

 ally sealed. For this reason food should, when possible, be alwavs preserved in glass 

 or china vessels. 



(FiG. 6.) 



\Vlii>rc germs Hourish and flies breed. 



(b.) Form the habit of examining each tin before opening it, as to the soundness of 

 its contents : 



(i.) By the Eyt. Slight inward bulging, or concavities, of the surface are a good 

 sign. Outward bulgings, or convexities, are a danger-signal, and show that 

 the gases of putrefaction have formed : 



