The coal burnt under the boiler vanishes without leaving a trace;
 so, too, the tallow with which the axles of wheels are greased. Dye stuffs
 and other auxiliary substances also vanish but re-appear as properties
 of the product. Raw material forms the substance of the product, but only
 after it has changed its form. Hence raw material and auxiliary 197substances
 lose the characteristic form with which they are clothed on entering the
 labour-process. It is otherwise with the instruments of labour. Tools,
 machines, workshops, and vessels, are of use in the labour-process, only
 so long as they retain their original shape, and are ready each morning
 to renew the process with their shape unchanged. And just as during their
 lifetime, that is to say, during the continued labour-process in which
 they serve, they retain their shape independent of the product, so, too,
 they do after their death. The corpses of machines, tools, workshops, &c.,
 are always separate and distinct from the product they helped to turn out.
 If we now consider the case of any instrument of labour during the whole
 period of its service, from the day of its entry into the workshop, till
 the day of its banishment into the lumber room, we find that during this
 period its use-value has been completely consumed, and therefore its exchange-value
 completely transferred to the product. For instance, if a spinning machine
 lasts for 10 years, it is plain that during that working period its total
 value is gradually transferred to the product of the 10 years. The lifetime
 of an instrument of labour, therefore, is spent in the repetition of a
 greater or less number of similar operations. Its life may be compared
 with that of a human being. Every day brings a man 24 hours nearer to his
 grave: but how many days he has still to travel on that road, no man can
 tell accurately by merely looking at him. This difficulty, however, does
 not prevent life insurance offices from drawing, by means of the theory
 of averages, very accurate, and at the same time very profitable conclusions.
 So it is with the instruments of labour. It is known by experience how
 long on the average a machine of a particular kind will last. Suppose its
 use-value in the labour-process to last only six days. Then, on the average,
 it loses each day one-sixth of its use-value, and therefore parts with
 one-sixth of its value to the daily product. The wear and tear of all instruments,
 their daily loss of use-value, and the corresponding quantity of value
 they part with to the product, are accordingly calculated upon this basis.