Lavoisier now studied the properties of the compounds produced by burning phosphorus, sulphur and carbon in dephlogisticated air. He found that solutions of these compounds in water had a more or less sour taste and turned certain blue colouring matters red; but these were the properties regarded as especially belonging to acids. These products of combustion in dephlogisticated air were therefore acids; but as phosphorus, carbon and sulphur were not themselves acids, the acid character of the substances obtained by burning these bodies in dephlogisticated air must be due to the presence in them of this air. Hence Lavoisier concluded that this air is the substance the presence of which in a compound confers acid properties on that compound. This view of the action of dephlogisticated air he perpetuated in the name "oxygen" (from Greek, = _acid-producer_), which he gave to dephlogisticated air, and by which name this gas has ever since been known. Priestley was of opinion that the atmosphere is rendered noxious by the breathing of animals, because it is thereby much phlogisticated, and he thought that his experiments rendered it very probable that plants are able to purify this noxious air by taking away phlogiston from it (see p. 69). But Lavoisier was now able to give a much more definite account of the effects on the atmosphere of animal and vegetable life. He had already shown that ordinary air contains oxygen and azote (nitrogen), and that the former is alone concerned in the process of combustion. He was now able to show that animals during respiration draw in air into their lungs: that a portion of the oxygen is there combined with carbon to form carbonic acid gas (as the fixed air of Black was now generally called), which is again expired along with unaltered azote. Respiration was thus proved to be a process chemically analogous to that of calcination.