they can be legitimately accounted for in accordance with the broader physico-chemical theory of fermentation, the truth of which has now been established.* We may also safely affirm that M. Pasteur's more specific statement, to the effect that the germs of some Bacteria and Vibriones are capable of resisting the influence of a heat of 212° F. when in the moist state, though they are killed by a temperature of 230° F., was a conclusion altogether unwarranted by the evidence which he adduced. Finding that certain fluids treated after the manner introduced by Schwann always remained quite devoid of living organisms, M. Pasteur very legitimately concluded that pre-existing organisms and germs had been killed during the boiling of the liquid; but finding that when a little powdered chalk was added to fluids of the same kind (which in all other respects were treated in a similar manner) living organisms were after a time invariably found to appear, although they as invariably failed to appear when the same fluids were heated to a temperature of 230° F. (110° C.), two equally legitimate provisional conclusions were open * I attempted to show, nearly three years ago (see Nature, July 14, 1870, pp. 224-228), that the differences which M. Pasteur ascribed to differences of vital resistance of organisms in particular fluids were just as explicable in accordance with the physico-chemical theory of fermentation, by reference to the different degrees of fermentability of the several fluids. to M. Pasteur in explanation of these facts. What did M. Pasteur do? Following the same method as he had formerly employed,* he again ignored one of the equally possible interpretations, and unsuccessfully attempted to prove, by a repetition of similar reasoning, that the different results in the two series of experiments were due to the fact that the germs of Bacteria and Vibriones which had been killed by the temperature of 212° F. in the first series were not killed by this temperature in the second series (in which a slightly alkaline fluid had been employed), although they were destroyed by the higher temperature of 230° F. Thus results which were due to the action of not-living ferments were ascribed to living ferments, and the possible action of not-living ferments was ignored, although, as I have said before, the ostensible object of M. Pasteur's researches was to inquire into the relative importance of not-living and living ferments, or whether, in fact, 'dead' substances (in the ordinary acceptation of the word) could act as ferments. When viewed from the stand-point of the physicochemical theory of fermentation, the apparently. contradictory results arrived at by the same experimenter at different times or by different experi * See note on page 113. + See "Ann. de Chim. et de Phys." 1862, pp. 60-65. menters, in this line of research, cease to be the inexplicable puzzle which they must always appear to those who place implicit faith in the narrower and too exclusive 'vital' theory of fermentation advocated by M. Pasteur and his followers. My investigations have convinced me that, with regard to degree of fermentability, the various fermentable fluids and mixtures are divisible into three distinct subclasses : I. There are what may be called self-fermentable fluids or mixtures-that is, fluids or mixtures which, after exposure to a temperature of 212° F. or higher, are still capable of undergoing fermentative changes without the addition of less-heated matter, either notliving or living. The changes occurring in these selffermentable fluids (in which pre-existing living things have been killed), when strictly protected from contact with adventitious particles, vary in rapidity and in intensity from the highest to the very lowest degrees of fermentability. These gradations are dependent principally upon the nature of the fluids or mixtures employed, and upon the degree of heat to which they have been submitted, though partly also to the temperature, pressure, presence or absence of filtered air, and degree of light to which the mixtures are subsequently exposed. For the sake of convenience, these gradations may be ranged into several distinct groups, though of course any such divisions as I am now about to sketch are purely artificial and are connected with one another in nature by innumerable transitions. Nature of Fluids. A. Turnip-infusion with cheese, turnip - infusion neutralized by liquor potassæ, ordinary turnipinfusion, strong hay-infusion, &c. B. Turnip-infusion neutralized by liquor potassæ, ordinary turnipinfusion, ordinary hay-infusion, &c. C. Beer-wort,* &c. D. Weak hay-infusions, urine, solutions containing ammonic carbonate and sodic phosphate with minute organic impurities, &c. Nature of Results. Within two to four days marked turbidity, owing to the appearance of swarms of Bacteria and Vibriones. Fluids more or less fœtid. (Putrefaction.) No uniform turbidity, but growth of flocculi in a more or less clear liquid. After a time the flocculi (composed of aggregated Bacteria and Vibriones) gradually subside, and the activity of the process ceases. Fluids either foetid or having a mere sour odour. Fluids which become more or less uniformly and rapidly turbid, owing to the appearance of swarms of Torulæ. Do not become visibly turbid or produce visible flocculi, although on microscopical examination they may be found to contain living Bacteria pretty uniformly distributed, but in comparatively small quantities. The odour is often not more appreciably altered than the clearness of such solutions. * I have had no experience with such a fluid myself. M. Pouchet's observations were, however, most striking on this subject (see his "Nouvelles Expériences,” Paris, 1864, p. 190). Nature of Fluids. E. Weak hay-infusions, urine, solutions containing ammonio-citrate of iron and minute organic impurities, &c. F. Solutions of ammonic tartrate and sodic phosphate with minute organic impurities, &c. G. Weak or strongly acid infusions, and also many saline solutions containing organic impurities. * Nature of Results. Same as in the last group, though Same as in the last group, only In solutions containing iron, green organisms may subsequently be found (see "Beginnings of Life,” vol. ii. p. 157). This, in fact, is in many cases the kind of change which the fluids last described ultimately undergo. See many negative experiments recorded in "The Beginnings of Life," vol. i. ch. xi. Mr. W. N. Hartley has laboured very industriously to disprove something which I never asserted (see Proceedings of Royal Society, vol. xx. p. 140). In my early paper in Nature, I expressly stated that organic impurities were always present in the saline solutions which I employed; and, as may be seen by the note appended to the conclusions of that paper, I never claimed to have |