eriment to express a real chemical transformation, true
both for the kinds of substances as well as for the weights, have any
value.

_Chemical formulas and equations, therefore, are a concise way of
representing qualitatively and quantitatively facts which have been
found by experiment to be true in reference to the composition of
substances and the changes which they undergo._

~Formulas representing water of crystallization.~ An examination of
substances containing water of crystallization has shown that in every
case the water is present in such proportion by weight as can readily be
represented by a formula. For example, copper sulphate (CuSO_{4}) and
water combine in the ratio of 1 molecule of the sulphate to 5 of water;
calcium sulphate (CaSO_{4}) and water combine in the ratio 1: 2 to form
gypsum. These facts are expressed by writing the formulas for the two
substances with a period between them. Thus the formula for crystallized
copper sulphate is CuSO_{4}.5H_{2}O; that of gypsum is CaSO_{4}.2H_{2}O.

~Heat of reaction.~ Attention has frequently been directed to the fact
that chemical changes are usually accompanied by heat changes. In
general it has been found that in every chemical action heat is either
absorbed or given off. By adopting a suitable unit for the measurement
of heat, the heat change during a chemical reaction can be expressed in
the equation for the reaction.

Heat cannot be measured by the use of a thermometer alone, since the
thermometer measures the intensity of heat, not its quantity. The
easiest way to measure a quantity of heat is to note how warm it will
make a definite amount of a given substance chosen as a standard. Water
has been chosen as the standard, and the unit of heat is called a
_calorie. A calorie is defined as the amount of heat required to raise
the temperature of one gram of water one degree._

By means of this unit it is easy to indicate the heat changes in a given
chemical reaction. The equation

2H + O = H_{2}O + 68,300 cal.

means that when 2.016 g. of hydrogen combine with 16 g. of oxygen,
18.016 g. of water are formed and 68,300 cal. are set free.

C + 2S = CS_{2} - 19,000 cal.

means that an expenditure of 19,000 cal. is required to cause 12 g. of
carbon to unite with 64.12 g. of sulphur to form 76.12 g. of carbon
disulphide. In these equations it will be noted that the symbols stand
for as many grams of the substance as there are units in the weig