In all these cases therefore the production time of the advanced capital consists of two periods: one period during which the capital is engaged in the labour-process and a second period during which its form of existence — that of an unfinished product — is abandoned to the sway of natural processes, without being at that time in the labour-process. Nor does it matter in the least here and there. The working period and the production period do not coincide in these cases. The production period is longer than the working period. But the product is not finished, not ready, hence not fit to be converted from the form of productive into that of commodity-capital until the production period is completed. Consequently the length of the turnover period increases in proportion to the length of the production time that does not consist of working time. In so far as the production time in excess of the working time is not fixed by natural laws given once and for all, such as govern the maturing of grain, the growth of an oak, etc., the period of turnover can often be more or less shortened by an artificial reduction of the production time. Such instances are the introduction of chemical bleaching instead of bleaching on the green and more efficient drying apparatus. Or, in tanning, where the penetration of the tannic acid into the skins, by the old method, took from six to eighteen months, while the new method, by means of an air-pump, does it in only one and a half to two months. (J. G. Courcelle-Seneuil, Traitè thèorique et pratique des entreprises industrielles, etc., Paris, 1857, 2-me èd.) The most magnificent illustration of an artificial abbreviation of the time of production taken up exclusively with natural processes is furnished by the history of iron manufacture, more especially the conversion of pig iron into steel during the last 100 years, from the puddling process discovered about 1780 to the modern Bessemer process and the latest methods introduced since. The time of production has been brought down tremendously, but the investment of fixed capital has increased in proportion.