The amusing features of the phlogiston theory only developed when it came to be realised that when the phlogiston was driven out of a body, as in the case of the calcination of a metal, the calx remaining was heavier than the metal with the phlogiston had been. The first explanation of this phenomenon was that phlogiston not only possessed no heaviness, but was actually endowed with a faculty of lightness. This hypothesis was, however, a little too far-fetched for even the seventeenth century. Boerhaave thereupon discovered that as the phlogiston escaped it attacked the vessel in which the metal was calcined, and combined some of that with the metal. This notion would not stand experiment, but Baume’s explanation of what happened was singularly ingenious. He insisted that phlogiston was appreciably ponderable. But, he said, when it is absorbed into a metal or other substance it does not combine with that substance, but is constantly in motion in the interstices of the molecules. So that as a bird in a cage does not add to the weight of the cage so long as it is flying about, no more does phlogiston add to the weight of the metal in which it is similarly flying about. But when the calcination takes place the dead phlogiston, as it may be called, does actually combine with the metal, and thus the increase of weight is accounted for. HUMOURS AND DEGREES. The doctrine of the “humours,” or humoral pathology, as it is generally termed, is usually traced to Hippocrates. It is set forth in his book on the Nature of Man, which Galen regarded as a genuine treatise of the Physician of Cos, but which other critics have supposed to have been written by one or more of his disciples or successors. At any rate, it is believed to represent his views. Plato elaborated the theory, and Galen gave it dogmatic form. The human body was composed not exactly of the four elements, earth, air, fire, and water, but of the essences of these elements. The fluid parts, the blood, the phlegm, the bile, and the black bile, were the four humours. There were also three kinds of spirits, natural, vital, and animal, which put the humours in motion. The blood was the humour which nourished the various parts of the body, and was the source of animal heat. The bile kept the passages of the body open, and served to promote the digestion of the food. The phlegm kept the nerves, the muscles, the cartilages, the tongue, and other organs supple, thus facilitating their movements. The black bile (the melancholy, Hippocrates termed it) was a link between the other humours and sustained them. The proportion of these humours occasioned the temperaments, and it is hardly necessary to remark that this fancy still prevails in our language; the sanguine, the bilious, the phlegmatic, and the atrabilious or melancholy natures being familiar descriptions to this day.