CHAPTER XXII
HEAT VACUUMS
IN THE preceding chapter we dealt with high temperatures and their employment in melting, molding, and working steel into useful forms. It will be well for us to pause here to consider temperatures at the other end of the thermometer scale, how they are obtained, and the important part they play in modern civilization.
It is not absolutely correct to speak of producing “cold.” We are apt to forget that cold is merely absence of heat. Strictly speaking, there is nothing cold on earth. Everything is more or less hot. A piece of ice at 32 degrees F. is hot compared with a lump of frozen alcohol, and the latter at its freezing point is hot compared with a lump of frozen air, while air at its freezing point is hot compared with a lump of solid helium. In other words, frozen alcohol will be melted by the heat in the ice; frozen air will be melted by the heat in frozen alcohol, and frozen helium will be fused by the heat in frozen air. Everything contains heat, and one object is colder than another only because it contains less heat.
Of course, the temperature of ice may vary. One block of ice may be ten, fifty, or a hundred degrees warmer than another, but ice cannot be heated above 32 degrees F. at the normal pressure of the atmosphere.
Ice is really a partial heat vacuum, a chamber partially exhausted, into which heat will flow if it gets a chance. We pack it away in sawdust, granulated cork, or other materials through which heat can with difficulty penetrate, and then in hot weather cakes of ice are placed in our household refrigerators, so that the heat that is in our food will have something to flow into. When we place our hands near a cake of ice, they feel cool and it seems as if ice radiated cold just as a stove radiates heat; but, of course, such is not the case. The heat of our hands radiates more rapidly in the direction of the cake of ice than in other directions, because there is a partial heat vacuum for the heat to flow into and the result is a sensation of cold.
We no longer depend upon cold winters for our supply of ice. We have learned how to pump heat and we can make heat vacuums, anywhere and at any time, even in the heart of the tropics, and regions in which no natural ice is ever obtainable have the benefits of refrigeration. Furthermore, we are not dependent upon ice for cooling foods. In many cases it is not necessary or even desirable to reduce temperatures to the freezing point of water. A moderate chilling is all that is required for certain foods. By the proper use of refrigerating machinery any degree of temperature may be obtained and maintained. To-day small refrigerating plants are constructed for domestic purposes, so as to render the housewife independent of the ice man.
With refrigerator cars and refrigerating plants on shipboard, fruit from the far west and from tropical lands may be brought to our breakfast table. Meats from northern slaughterhouses may be transported in perfect condition into hot southern climes. There are also certain industries which are dependent on the use of the low temperatures. In breweries, dairies, margarine factories, etc., refrigeration is of the utmost importance, and refrigerating machinery is used for cooling and drying the air blast for blast furnaces.