Thus the rules laid down by Pasteur and others are not universal, and therefore, as I have previously pointed out, the explanation which he adduced of the preservation of those particular fluids which remained unchanged is at once rendered doubtful. More especially is there room for doubt on this subject when, as I have found, the result of the experiment can be, within certain limits, predicated beforehand, according to the nature of the fluid employed. If all organisms proceed from pre-existing germs, and these can be filtered from the air by a certain mechanical contrivance, then, if it be alleged that it is on account of such filtration that certain boiled fluids do not change, all fluids placed under these conditions ought, on this theory, to be similarly preserved. Exceptional cases cannot be accounted for on this hypothesis. To others, however, who say that organisms are capable of arising _de novo_, and that fermentation can be initiated without the agency of living things, the above facts appear quite natural. The more complex the nitrogenous or protein materials contained in a solution, the more is it fitted to undergo fermentative changes, which may be accompanied by the _de novo_ origination of living things. Therefore the above results are just as compatible with the notions of M. Liebig and his school, as they are antagonistic to those of M. Pasteur. Certain fluids, it is found, do not undergo change; whilst other fluids, of a more complex description, will ferment under the influence of similar conditions. Prolonged ebullition also, by breaking up some of the more unstable compounds of a solution (those which most easily initiate these changes) will retard or prevent its fermentation. The complete untenability of M. Pasteur’s explanations are, however, best revealed by having recourse to a series of comparative experiments, in which portions of the same fluid are boiled for an equal length of time in vessels of different kinds, and are then subsequently submitted, in a water-bath, to the influence of the same temperature. I have made many experiments of this kind with different solutions, some of which I will now record. Owing to the different behaviour of the same fluids under different conditions, we are enabled to draw some most important conclusions; and owing to the different behaviour of different fluids under these respective conditions, our attention is strongly drawn to other facts which ought considerably to influence our judgment as to the relative merits of the two doctrines concerning the cause of fermentation and putrefaction. COMPARATIVE EXPERIMENTS. In the following experiments, each fluid (unless a statement is made to the contrary) was _boiled continuously for ten minutes_, after having been placed in its flask. Then, with the neck either open, sealed, or plugged, the bulb of the flask was immersed in a water-bath maintained at a temperature of 80°–95° F., during both day and night.[47]