93 MODERN FRUIT MARKETING 
unit of measure for heat is the calorie. This equals 
the heat required to raise one kilogram of water from 
zero to one degree Centigrade. 
Then, in measuring the transmission of heat through 
various insulating materials, calculations are based upon 
the number of B. T. U. that will pass through one square 
foot of substance per hour per degree, difference in 
temperature between the two sides of the substance. 
This enables definite experiments to be carried on and 
Fig. 84.—CORK BOARD 
the insulating value of different materials can be def- 
initely determined. Substances which allow heat to 
pass through readily are known as good conductors of 
heat. Those which retard the passage of heat are poor 
conductors. Hence, for insulating storage houses against 
heat and cold we always select poor conductors. 
Relative Heat-Conducting Power.—From a number 
of experiments that have been made both in the United 
States and Europe, the following figures have been taken. 
The relative insulating value of the different ma- 
terials are based on water as a standard. As physicists 
use water on a basis of one for the specific gravity of 
other liquids or solids, so in testing for insulating value 
