50 BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 
in others, so that small pressures are to be expected. The amount of | 
pressure actually developed will depend, of course, upon the perfec- — 
tion in construction of the container, the thickness and quality of the — 
rubber rings, and the pressure ete by the device holding the cap 
in place. These pressures are practically negligible. 
Investigations upon the resistance of rubber rings and other sealing 
equipment designed for use with glass are receiving attention at the 
hands of the manufacturers of glass containers, and Bitting (6) has 
recently published the partial results of work upon this subject. 
The fact that with some substances an undesirable metallic taste is 
imparted to the food by the tin makes the use of glass containers de-_ || 
sirable; and knowledge of the fact that, with fruits especially, deli- 
cate flavors may be retained in the product by completely sealing the 
jar before processing and then cooking at temperatures below 100° 
C: will doubtless lend impetus to investigations of this sort and result 
in the manufacture of glass containers better suited than those now 
employed for use where pressures are developed. 
On account of their rigidity the pressures and vacuums obtained in 
glass jars hermetically sealed before processing should give figures 
more closely approaching the theoretical values than those obtained 
with tin. Owing to the fact that, in present practice, most, if not all, 
of the air escapes during the processing, the vacuum obtained is close 
to the theoretical value when the jars are sealed immediately at the 
end of the processing period. Unless complete sealing is effected at 
once, however, air is drawn into the jar and the vacuum obtained is 
less, qependine upon the rate of cooling and the length of time 
insane before the jar is sealed. 
| SUMMARY. 
(1) In tin cans containing various quantities of water, changes 
in pressure vary somewhat from the calculated values, owing to the 
distortion of the cam under the changed conditions. 
(2) With water the rate of change of pressure and the rate of 
change of temperature at the center of the can agree closely and are 
very rapid where the external medium is water and very slow where 
the external medium is air. 
(3) With food materials in which a free liquid fills the inter- 
spaces the rate of change of pressure and of temperature is very 
rapid; but while the maximum temperature is reached promptly, 
the maximum pressure, on the other hand, is never reached during 
the ordinary processing periods, the pressure continuing to rise 
slowly as long as the high retort temperatures are maintained. 
(4) In cans filled sel material of heavy consistency, the rate 
of change of temperature at the center of the can is very slow. In 
contrast with this, the rate of change of pressure is very rapid at 
