other peculiarities, the chief of which are the dependence of all its activities upon moisture and upon heat, within a limited range of temperature, and the fact that it usually possesses a certain structure, or organization.
Life conditioned by moisture.As has been said, a large proportion of water enters into the composition of all living matter; a certain amount of drying arrests vital activity, and the complete abstraction of this water is absolutely incompatible with either actual or potential life. But many of the simpler forms of life may undergo desiccation to such an extent as to arrest their vital manifestations and convert them into the semblance of not-living matter, and yet remain potentially alive. That is to say, on being duly moistened they return to life again. And this revivification may take place after months, or even years, of arrested life.
Life conditioned by temperature.The properties of living matter are intimately related to temperature. Not only does exposure to heat sufficient to decompose protein matter destroy life, by demolishing the molecular structure upon which life depends; but all vital activity, all phenomena of nutritive growth, movement, and reproduction are possible only between certain limits of temperature. As the temperature approaches these limits the manifestations of life vanish, though they may be recovered by return to the normal conditions; but if it pass far beyond these limits, death takes place.
This much is clear; but it is not easy to say exactly what the limits of temperature are, as they appear to vary in part with the kind of living matter, and in part with the conditions of moisture which obtain along with the temperature. The conditions of life are so complex in the higher organisms, that the experimental investigation of this question can be satisfactorily attempted only in the lowest and simplest forms. It appears that, in the dry state, these are able to bear far greater extremes both of heat and cold than in the moist condition. Thus Pasteur found that the spores of fungi, when dry, could be exposed without destruction to a temperature of 120°-125° C. (248°-257° Fahr.), while the same spores, when moist, were all killed by exposure to 100° C. (212° Fahr.) On the other hand, Cagniard de la Tour found that dry yeast might be exposed to the extremely low temperature of solid carbonic acid (-60° C. or -76° Fahr.) without being killed. In the moist state he found that it might be frozen and cooled to -5° C. (23° Fahr.), but that it was killed by lower temperatures. However, it is very desirable that these experiments should be repeated, for Cohn's careful observations on Bacteria show that, though they fall into a state of torpidity, and, like yeast, lose all their powers of exciting fermentation at, or near, the freezing point of water, they are not killed by exposure for five hours to a temperature below -10° C. (14° Fahr.), and, for some time, sinking to 18° C. (-0°·4 Fahr.) Specimens of Spirillum volutans, which had been cooled to this extent, began to move about some little time after the ice containing them thawed. But Cohn remarks that Euglenæ, which were frozen along with them, were all killed and disorganised, and that the same fate had befallen the higher Infusoria and Rotifera, with the exception of some encysted Vorticellæ, in which the rhythmical movements of the contractile vesicle showed that life was preserved.
Thus it would appear that the resistance of living matter to cold depends greatly on the special form of that matter, and that the limit of the Euglena, simple organism as it is, is much higher than that of the Bacterium.
Considerations of this kind throw some light upon the apparently anomalous conditions under which many of the lower plants, such as Protococcus and the Diatomaceæ, and some of the lower animals, such as the Radiolaria, are observed to flourish. Protococcus has been found, not only on the snows of great heights in temperate latitudes, but covering extensive areas of ice and snow in the Arctic regions, where it must be exposed to extremely low temperatures, in the latter case for many months together; while the Arctic and Antarctic seas swarm with Diatomaceæ and Radiolaria. It is on the Diatomaceæ, as Hooker has well shown, that all surface life in these regions ultimately depends; and their enormous multitudes prove that their rate of multiplication is adequate to meet the demands made upon them, and is not seriously impeded by the low temperature of the waters, never much above the freezing point, in which they habitually live.
The maximum limit of heat which living matter can resist is no less variable than its minimum limit. Kühne found that marine Amœbæ were killed when the temperature reached 35° C. (95° Fahr,), while this was not the case with fresh-water Amœbæ, which survived a heat of 5°, or even 10°, C. higher. And Actinophrys Eichornii was not killed until the temperature rose to 44° or 45° C. Didymium serpula is killed at 35° C.; while another Myxomycete, Æthalium septicum, succumbs only at 40° C.
Cohn ("Untersuchungen über Bacterien," Beiträge zur Biologie der Pflanzen, Heft 2, 1872) has given the results of a series of experiments conducted with the view of ascertaining the temperature at which Bacteria are destroyed, when living in a fluid of definite chemical composition, and free from all such complications as must arise from the inequalities of physical condition when solid particles other than the Bacteria co-exist with them. The fluid employed contained 0·1 gramme potassium phosphate, 0·1 gr. crystallised magnesium sulphate, 0·1 gr. tribasic calcium phosphate, and 0·2 gr. ammonium tartrate, dissolved in 20 cubic centimetres of distilled water. If to a certain quantity of this "normal fluid" a small proportion of water containing Bacteria was added, the multiplication of the Bacteria went on with rapidity, whether the mouth of the containing flask was open or hermetically closed. Hermetically-sealed flasks, containing portions of the normal fluid infected with Bacteria, were submerged in water heated to various temperatures, the flask being carefully shaken, without being raised out of the water, during its submergence.
The result was, that in those flasks which were thus subjected, for an hour, to a heat of 60°-62° C. (140°-143° Fahr.), the Bacteria underwent no development, and the fluid remained perfectly clear. On the other hand, in similar experiments in which the flasks were heated only to 40° or 50° C. (104°-122° Fahr.), the fluid became turbid, in consequence of the multiplication of the Bacteria, in the course of from two to three days.
Both in Kühne's and in Cohn's experiments, which last have lately been confirmed and extended by Dr Roberts of Manchester, it was noted that long exposure to a lower temperature than that which brings about immediate destruction of life, produces the same effect as short exposure to the latter temperature. Thus, though all the Bacteria were killed, with certainty, in the normal fluid, by short exposure to temperatures at or above 60° C. (140° Fahr.), Cohn observed that, when a flask containing infected normal fluid was heated to 50°-52° C. (122°-125° Fahr.) for only an hour, the consequent multiplication of the Bacteria was manifested much earlier, than in one which had been exposed for two hours to the same temperature.
It appears to be very generally held that the simpler vegetable organisms are deprived of life at temperatures as high as 60° C. (140° Fahr.); but Algæ have been found living in hot springs at much higher temperatures, namely, from 168° to 208° Fahr., for which latter surprising fact we have the high authority of Descloiseaux. It is no ex-
