176 BUTTER-MAKING. 
(5) Proper utilization of steam turned into the pasteurizer. 
(1) Heating-surface—It is a well known fact that some 
metals will conduct heat better than others. The relative heat 
conductivity of the two substances used chiefly for pasteurizers, 
viz., copper and tin, is .918 and .145 respectively. This 
means that copper will conduct heat nearly seven times faster 
than tin of the same thickness. 
= —— 
ne ir 
Fig. 116.—The Reid pasteurizer. 
In connection with this it should be mentioned that stability 
and durability of the substance must also be taken into con- 
sideration. A heating-surface made from copper may be nearly 
seven times as thick as that made from tin, and still transmit 
as much heat as the tin surface. From this it can be seen that 
a heating wall made from copper can be increased slightly in 
thickness, and thus aid in stability, without affecting the degree 
of heat conductivity of the wall very much. The heating- 
surface must be strong enough to withstand a slight steam pres- 
sure, otherwise the heating wall is likely to collapse or cave 
in in case of slight variation in the steam pressure. It is not 
