THE BAKERS GUIDE

it receives the concentrated thought of research. This modern dynamical theory says that heat is not matter, but only a con- dition of matter and a motion of its ultimate particles — that heat is a brisk agitation of the insensible parts of the object which produces in us that sensation, from whence we denominate the object hot. So that what in our sensation is heat, in the object is nothing but motion.

All heat is of the same kind, whether produced by burning of fuel, or by mechanical means (friction or force applied to a body), or by the change of state in the passage of a less stable body to a more stable arrangement of atoms, whether the change is produced by a chemical combination or decom- position. Molecular changes are nearly always attended with evolution or absortion of heat. But though all heat is of the same kind it is not always of the same quantity. (A half- sovereign is gold, but there is not so much gold in it as in a sovereign).

This brings the subject to the difference there is between the thermal state of a body and the quantity of heat there is in it. Take as an example 1 lb. of water at ioo°c, and 2 lbs. at the same temperature. It must be obvious to you that though both are the same in temperature, there must be a larger quantity of heat in the 2 lbs. than in the 1 lb. Very briefly I will explain this. The temperature of a body is a measure of the degree of its heat. , This is the work of the thermometer. The measure of the quantity of heat in a substance is the number of heat units there are in it, what I do mean by units of heat, all measurements of the quantity of anything must have a standard unit to start from. The standard unit in the measure- ment of the quantity of heat is the quantity of heat necessary to raise 1 gram of water 1 degree from o° c. to i° c. To see how much heat, therefore, there is in the 1 lb. of water it is only necessary to find out how many grams it contains — which is 373. The heat of this lb. of water was ioo° and therefore this 373 must be multiplied by 100, viz., 37,300 heat units are in it, and of course double that number in the double quantity of 2 lbs. at the same degree of heat. The weight of water in grams, multiplied by the number of the degrees of temperature through which it is raised equals the number of H. U. (heat units) in it.

Hot and Cold are terms generally used, but they are merely different degrees of the same thing — Heat.

Sensible or free Heat is that which produces the sensation of heat, or affects the thermometer.

Latent Heat is that portion which passes into bodies during a change of form, without elevating their temperature ; as into ice