34 THE ECONOMICS OF FEEDING HORSES 



that these food elements have in supplying the wants 

 of the animal. The only satisfactory way to estimate 

 this is in terms of their heat-producing" power when 

 broken down in the tissues. Proteins, fats, and 

 carbo-hydrates, when fully combined with oxygen, are 

 split up much in the same way that combustible 

 material is burned in a fire, and with a similar result 

 — namely, the production of heat. This heat is capable 

 of conversion into various other forms of energy, such 

 as the muscular energy expended in locomotion ; in doing 

 work, as in the case of a draught-horse ; in carrying 

 on the vital bodily processes, such as the movements 

 of the heart and respiratory organs ; and in the secretion 

 of the juices from the various glands. All these forms 

 of energy can be estimated and stated in terms of heat, 

 and all are provided for by the food and air taken in 

 by the animal. Thus the heat-producing capacity of 

 a food is the real measure of its nutritive and work- 

 producing value. In other words, there is a direct 

 proportion between the amount of heat produced by 

 oxidising or burning the nutritive parts of a food 

 and the amount of energy or labour they will supply 

 after assimilation by a horse or other animal. Many 

 and repeated experiments have been made to ascertain 

 the comparative heat-producing power of the nutritive 

 constituents of food. The principle made use of in 

 these tests is to take a certain amount of the substance 

 under investigation, mix it with an excess of some active 

 oxidising agent, such as potassium chlorate, and place 

 the mixture in a metal case or bomb, provided with 

 electric terminals. The bomb is inserted in a special 

 form of water calorimeter (a heat-measuring instrument), 

 and by means of an electric spark the mixture is fused. 



