It is a well-established fact that when the same weight of the same substance burns to form the same products of combustion, the quantity of heat evolved is a constant, the value of which is independent of the rate of combustion. This quantity is known as the heat of chemical combination of the substance with oxygen. In relation to substances used as fuels, it is also known as the "calorific value." Thus 1 gram of carbon burning to carbon dioxide evolves 8,080 gram-calories of heat, and 1 gram of hydrogen burning to liquid water evolves 34,462 gram-calories (Favre and Silbermann). The calorific value of carbon is therefore said to be 8,080, and of hydrogen 34,462. The gram-calorie is defined as the quantity of heat required to raise the temperature of 1 gram of water by one degree Centigrade.

When alcohol is completely burned in air or oxygen, the products of combustion are water and carbon dioxide. The heat evolved, i.e., the calorific value can be calculated theoretically from the data just given, as shown below.

The percentage composition in ethyl alcohol is: Carbon 52.13, hydrogen 1312, and oxygen 3475. Hence 1 gram of ethyl,alcohol contains: -

Deducting from the amount of hydrogen the quantity necessary to form water with the oxygen, i.e.,0.3475/8= 0 0434 gram, we have

01312 - 00434=00878.

Multiplying by the corresponding calorific values for the carbon and hydrogen and adding the products, we have: -

1 "Fuel," p. 158 (1914 edition).

The calculation here shown at length may be summarised into the formula: -

Calorific value = 808 0 + 3446 (H - 0/8), where C, H, and 0 denote the percentages of carbon, hydrogen, and oxygen respectively.

A somewhat higher result is given if the values for carbon and hydrogen used in Berthelot's formula are taken - namely, 8,137 and 34.500, instead of those employed in the calculation shown. On the other hand, in summary calculations the round numbers 8,000 and 34,000 are often used.

In practice, of course, absolute alcohol is not employed as fuel; but the calculation shows the principle most simply, and the result is required in obtaining the theoretical calorific value of denatured alcohol, and of other alcohol mixtures, which are used as fuels.

In a similar manner, the theoretical calorific value of absolute methyl alcohol, the percentage composition of which is C 37.47, H 1258, and 0 4995, is found to be 5,2125. Acetone, C3H60, has the calculated value 7,4865: and benzene, C6H6, the value 10,111 4. Petrol is a variable mixture of hydrocarbons, but if taken as being substantially equivalent to octane, C8H18, its theoretical calorific value is 12,245 calories per gram.

"Gross" and "net" values. - The values here given are the " higher" or " gross" calorific values, corresponding with the formation of liquid water, as obtained in the calorimeter. If the water is vaporised, this operation absorbs some of the heat developed by the combustion. The quantity of water produced is governed by the amount of hydrogen burned. Thus in the case of ethyl alcohol the weight of hydrogen in 1 gram is 01312 gram, and this is equivalent to 1.808 grams of water. Since the latent heat of vaporisation of water is 537 calories per gram, the heat absorbed in vaporising this water is 11808 x 537 = 634. calories. This quantity, therefore, is to be deducted from the result obtained above for ethyl alcohol, whenever the " lower" or "net" value is required: 7,2378 - 6341 = 6,6037, the lower theoretical calorific value of absolute ethyl alcohol.

Further, if the alcohol in question is not absolute, but already contains water (as is always the case in practice), the vaporisation of this water will also absorb heat, and the corresponding number of calories must be deducted. Thus if the proportion of water be 10 per cent. by weight, 1 gram of the alcohol will contain 01 gram of water, and the further deduction will therefore be 01 X 537 = 53.7 calories.

Hence, finally, for 90 per cent. alcohol (by weight), when all the water already present and that produced in the combustion is vaporised, the calorific value is 6,6037 - 537 = 6,5500 gram-calories.