44 MADRAS FISHERIES BULLETIN VOL. XI IT, 



etc., and thus sealed almost or quite hermetically. Screw tops are, 

 except for glass jars, unsuitable or unnecessary for cans that have 

 to be sealed hermetically and subsequently processed. 



20. Glass jars. — This elegant method of canning is practically, 

 if not entirely, unknown in India, but is common elsewhere, and, 

 until the war, was annually growing in favour. The advantages 

 are — 



(1) hygienic, since glass is not acted upon either by the 

 constituents of animal tissues especially those of shell fish, or by 

 the acids of fruits ; hence there is no danger of forming poisonous 

 metallic compounds ; moreover the condition of the goods can be 

 ascertained by inspection and bad goods (e.g., mushy fruits) 

 rejected : 



(2) aesthetic, since not only can glass jars be made of pleasing 

 form and appearance, but the goods, especially fruits, clear marma- 

 lades or jellies, whole fish roes and milts, etc., can be packed in a 

 very attractive manner which of itself induces custom ; 



(3) economic, since the glass jars can be used over and over 

 again, or can be used for other domestic purposes ; moreover these 

 jars are adapted for household canning since no soldering is 

 required, and no plant except that found in most kitchens. 



The disadvantages are their comparatively great weight, danger 

 of breakage especially during manufacture if exposed suddenly to 

 heat or cold air, and during transport, cost as compared with tins, 

 and greater susceptibility to air leakage. 



In the case of glass jars hermetical sealing is obtained wholly 

 by atmospheric pressure ; the jars are filled with the product to be 

 canned, lightly closed with a special cap and heated to the desired 

 degree so that the air is expelled and steam produced; on the 

 cessation of boiling the steam condenses leaving a vacuum, so that 

 the cap is forced down on the mouth of the jar by atmospheric 

 pressure. The special caps are of several different forms, but 

 always contain a washer of rubber or composition which fits 

 accurately on the mouth of the jar which is made flat for the pur- 

 pose. This washer closes the mouth air-tight and is kept in 

 position by the cap which is either screwed down or clamped by a 

 crimping machine under a shoulder in the neck of the jar. In a 

 fish-canning factory in the United States of America, jars are 

 placed in a vacuumising receiver, the air is thoroughly exhausted, 

 and then let suddenly in so that the cap and washer are driven 



