REFRIGERATION OF DRESSED POULTRY IX TRANSIT. 19 
proof under the continual stress and strain to which refrigerator 
cars are subjected. Cracks in the hoards and punctures in the 
paper lining soon appear and, with the resulting air circulation, heat 
is directly transferred from the outside to the inside. A heat insu- 
lator is a nonconductor of heat. Heat is a form of energy trans- 
ferred in waves of extremely small length from one molecule to 
another. Since the molecules in solid bodies are closer together than 
those of gases, the solids are the better heat conductors. The more 
numerous the air spaces in a solid body the more efficient it will be 
as an insulator. 
Cork, the best known insulator, contains innumerable air spaces, 
and its texture renders it almost impervious to water. It contains 
but small amounts of gums and resins and practically no nitrogenous 
material which might serve as a medium for bacterial growth and 
thus produce decay. Cork, however, has not been used to any 
extent in car construction, perhaps on account of its expense and 
the difficulty of its application. 
Wool a'nd hair felt are good insulators as long as they are kept dry, 
but their high percentage of nitrogenous material makes them good 
bacterial media when moist. Organic oils and acids also aid in their 
decomposition. These materials, when once moist, seldom dry out, 
and the result is putrefaction, giving rise to offensive odors, which 
contaminate the goods in the car. This decomposition not only 
destroys the insulator itself but rots the board lining with which it 
comes in contact. Some of the vegetable 01 cellulose fiber insulators 
are perhaps sUghtly more resistant to moisture and bacterial action, 
but in time they also become moist and their chemical decomposition 
is hastened by the alkalies present in such material. Of the insu- 
lators mentioned, mineral wool is the least subject to decay, but, 
on the other hand, its physical nonadhesive properties hinder the 
manufacture of strong material, and its insulating qualities are not 
as good as those of some of the other nonconductors, although it has 
the advantage of being fireproof. 
Careful consideration of insulation is therefore one of the prime 
factors in car construction. The material must be of such a nature 
that it will remain in position, not settling down and leaving hollow 
spaces in the upper portion of the side walls. It should be impervious 
to moisture, or be securely protected by moisture-proof material, and 
as free as possible from decomposable organic matter. The neces- 
sary thickness of the insulation depends on the nature of the goods 
to be transported. Investigations in fruit transportation have shown 
that temperatures as low as 40° F. are very satisfactory for citrus 
fruits, but the results with poultry indicate that lower temperatures 
are essential for a maximum preservation of this class of goods; 
Aside from the nature of the lading, the question of insulation is one 
