It is not the fact that absolutely all chemical union and change whatsoever is prevented–that is to say, arrested or suspended–by extreme cold, although the union with oxygen and other such changes of the essential material of living things, which we call "protoplasm," and most other chemical changes are thus arrested or suspended. The most striking exception is that of the most active of all elements, the gas fluorine, which becomes a liquid at 210 degrees below zero centigrade, and in that condition attacks turpentine if brought into contact with it at the same low temperature with explosive force. Even solid fluorine combines with liquid hydrogen with violent explosion. It seems certain, however, that elements or chemical compounds brought into the solid (not merely liquid) condition by extreme cold cannot act chemically upon other bodies in the same solid condition, even when they would at normal temperatures so act with the greatest readiness, because they are then either liquid or gaseous. The conception of an arrest of the changes in organisms, which we call life, followed by their resumption after a greater or less interval of suspense, was long ago suggested and discussed before we had knowledge of the action of low temperatures. The winter-sleep of some animals and the "comatose" condition sometimes exhibited by human beings had led to the notion of "suspended animation." But a careful study of hybernating animals and of human instances of prolonged "coma" satisfied physiologists nearly 100 years ago that the processes of life–the beating of the heart and the respiration–were not actually and absolutely suspended in these cases, but reduced to a minimum. The chemical processes connected with life were still very slowly carried on. Again, a great deal of interest and discussion was excited in the last century by the drying up of delicate yet complex aquatic animalcules, such as the Rotifers (the wheel animalcules described in our last chapter) and Tardigrades (bear animalcules), and the fact that after their preservation as mere dust for many months dried on a glass-slip they could be revived and made to return to life by wetting them with a minute drop of water, whilst the whole process of revival was watched under the microscope. Letters were published in the "Times" in the "fifties" by the Rev. Lord Sydney Godolphin Osborn, describing his observations and experiments on these animalcules.