TRANSPORTATION PROBLEMS. 175 
c-xtcnt of leading the consumer to believe he can keep it indefinitely. 
The consumer is thus forced to use it up before the spore-bearing 
bacteria get an opportunity of multiplying sufficiently to produce 
the injurious secretions which "occasionally render sterilized milk 
dangerous. The very fact that the method does not destroy all 
bacteria is a safeguard. 
3. It removes the danger of distributing pathogenic bacteria. 
This is certainly quite true of the typical diseases mentioned. 
\Yhether it similarly removes the danger of diarrheal diseases, 
not dependent upon any known specific bacteria, is not yet positively 
known by experiment, inasmuch as we do not know the actual 
cause of the diseases. But the practical experience of physicians 
tells us that pasteurized milk acts as efficiently as sterilized milk 
in reducing these diseases. 
4. This method of treatment is perfectly applicable upon a 
large scale. Several forms of apparatus have been devised that 
accomplish the end rapidly and upon large quantities of milk. 
Of these, there are two general types. In one a large quantity of 
milk is heated to the desired temperature and maintained at this 
temperature as long as desired, after which it is cooled. These 
are called discontinuous pasteurizers. In the other type the milk 
is passed through the apparatus in a constant stream, being heated 
and cooled while it passes through. In these machines the milk 
is sometimes only just brought to the desired temperature, and 
cooled at once; and in all cases the extent of the heating is dependent 
upon the rapidity of the stream flowing through. These are called 
continuous pasteurizers. Generally speaking, this type is apt to 
be less efficient than the discontinuous pasteurizers, and are more 
subject to irregularity. Either type is efficient if properly managed, 
but carelessness and haste on the part of the employees may render 
either kind unreliable and inefficient. 
In the last few years the plan of pasteurizing the milk on a large 
scale has come to be frequently adopted. It is done in creameries 
in connection with butter-making, and in some of our large cities 
for the treatment of the general milk-supply. In the pasteuriza- 
tion of the public milk-supply the purpose has not been, primarily, to 
