After having referred to a previous study of the equations proposed by several workers for calculating the calorific value of the coals from the proximate analysis data, in which study - through the application of those formulas to a set of 90 Spanish coals - the necessity to introduce a correction factor was proved, it is shown that this factor must take into account the following points: a) the gravimetric difference between the ashes and the mineral matter, primarily present in the coal, and b) the thermic tonality of the reactions which take place during the combustion of that mineral matter. As for the first of these two considerations, it is verified: 1SI) That the hydration water content of clays and shales composing a part of the mineral matter of Spanish coals is, as an average, 8 0/0' this referred to the weight of the calcined clay. To determine the clays content of a coal as well as to research the hydration water of those, original processes are described. 2 nd) That the formula suggested by King, Maries and Crossley to calculate the mineral matter of English coals is appropriate to those of Spain. 3 rd ) That the factor by which the ash content have to be multiplied in order to transform it into mineral matter, may be valued as 1-15. As regards to the thermic tonality of the transformation of mineral matter into ash, the most important term is the one due to pyrite combustion. In the same way, the heat developed on the reactions between carbonates and the strong acid atmosphere inside the calorimeter bomb, as well as the heat consumed in the dehydration of clays, have been taken into account. Since the sum of these terms is a positive one, this heat produced by combustion of mineral matter partly compensates the correction due to the gravimetric difference between ashes and mineral matter, As an average, it is equivalent to a decreasing of 0•03 in the factor which correc1ed that difference, why, in connection with the calculation of the calorific value, ilie ash content must be multiplied by 1-12, in order to compensate both sources of error. It is deduced from the paper the advantage to consider the calorific value of coals as a function not only of the volatile matter content but also of any other parameter which allows to classify fuels into several ranks, so differing coals with identical volatile contents and very different properties. Owing to its ease and facility of determination, the crucible swelling index has been taken into account~ by means of which the Spanish coals were divided. into four classes, the calorific values of which, as a function 01 the volatile matters content, are grouped in sensible parabolic ):lands. The corresponding curves have been calculated statistically and their values applied through the formula P = Q (100 - 1•12 A) to the same set of 90 coals utilized for studying another expressions of the calorific value getting results remarkably more consistent with the values given by the bomb.

Peer reviewed