836 WATER HYGIENE 
filled with distilled water; if ordinary water with a high mineral content 
is used, the cake of ice, after it has been frozen, will have a white core 
through the center. This is formed from the dissolved and suspended 
matter and will also contain the bacteria which were present in the 
original water. 
Properties of Ice. Ice is crystallized water which is made to form 
by the removal of heat. The temperature at which this change from 
the liquid to the solid phase takes place is dependent upon a number 
of different factors. The presence of dissolved air, inorganic salts 
pressure, etc., are all concerned. It is well known that salt water 
freezes at a lower temperature than pure water. During the freezing 
process, the water contracts until it reaches 4° C. when it begins to 
expand and becomes lighter than water. Since the formation of ice is a 
process of crystallization, it naturally follows the laws of crystallization. 
Substances which crystallize from solution tend to come out in the pure 
state. The chemist uses this method for purifying his reagents. The 
foreign matter is excluded unless a small amount may be occluded 
during the process between the crystals. The same thing takes place 
when. water crystallizes. The bacteria and suspended matter are 
forced out and do not appear in the ice. Ice made from dirty water by 
the can method usually shows the dirt in a core. This will be found 
to contain many bacteria; more are found in this core than in the ice 
at the surfaces of the can. 
Bacteriology of Ice. Ice does not furnish the essentials of a good 
medium and consequently there is no multiplication of bacteria. The 
question resolves itself into determining how many bacteria may sur- 
vive in such an environment. The following causes contribute to the 
inhibitory action on bacteria: 
1. Temperature. 
2. Lack of oxygen. 
3. Absence of moisture. 
The longevity of bacteria in ice has received some attention from 
bacteriologists and sanitarians. Frankland (1894) reports some data 
which were secured by Prudden. Culture of Bacillus prodigiosus, 
Bacillus proteus vulgaris, Staphylococcus pyogenes aureus a fluorescent 
bacillus and Bacillus typhosus were put into samples of water and 
exposed to temperatures of from 14° to 30° F. for 103 days. Bacillus 
prodigzosus decreased from 6300 in 4 days to 3000; in 37 days the number 
went to 22 and after 51 days all of the cells had died. Bacillus proteus 
vulgaris disappeared in 51 days. At the end of 11 days there were 
