For the discovery of the composition of atmospheric air, we are indebted to Scheele, an able chemist, born 1742, at Stralsund, in Germany, who was a member of the Academy of Stockholm, and one of the Royal Society of Medicine at Paris, and whose laborious investigations of nature have perpetuated his memory. When the nature of atmospheric air began to be understood, it was imagined that it was a mere _mixture_ of oxygen gas and nitrogen gas; and Mr. Dalton is still of this opinion: but, says Mr. Parkes, we have now abundant reason to believe that it is a mere chemical compound; that is, that the oxygen and nitrogen form atmospheric air by a chemical union. Atmospheric air is a chemical mixture of oxygen and nitrogen rendered aërial by the expansive power of caloric: it likewise contains a portion of carbonic acid gas, which was formerly calculated at one per cent.; but Mr. Dalton has lately demonstrated that it does not amount to more than one part in a thousand.[57] Carbonic acid gas is nearly twice as heavy as common air; hence it is evident that it must combine _chemically_ with the atmosphere, or it would be found only near the surface of the earth. If it were merely _mixed_ with atmospheric air, its gravity would prevent it from ascending to any great height: but it is found to exist in the atmosphere at the greatest heights, (though probably not in the same proportion) as well as near the surface of the earth; which is a proof that it is not a mere mixture, but that it is chemically combined with the air. There are about 22 parts of oxygen, and 78 of nitrogen, in every 100 measures of atmospheric air, or 23 of the former and 77 of the latter, if the calculation be made by weight.[58] Antony de Marti observes, If a few hundredth parts of oxygen only were wanting in atmospheric air, fire would lose its strength, candles would not diffuse such complete light, and animals would with difficulty separate the necessary quantity of the vivifying oxygen. On the other hand, if the atmosphere were more charged with oxygen than nitrogen, animals indeed would acquire a more free respiration; but, let us consider the activity which fire would acquire by air of superior purity. We know that, on some occasions, the least spark excites the strongest flame in a combustible body, and which increases so much as to consume it in a few moments: candles _then_ would be no sooner lighted than they would be destroyed, without answering any other purpose than that of dazzling us for a few moments: iron would be calcined, instead of acquiring from the fire that softness necessary for transforming it into its various instruments, and which it cannot receive in a more moderate heat. Nothing would be capable of checking the progress of this destructive element, which is nourished by vital air, if this aëriform substance were not abundantly mixed with mephitic air, which serves to restrain it.