It remains now to take into consideration the pneumatic theories, founded upon the discoveries made by Dr. Black, Dr. Priestley, Lavoisier, and others, concerning various kinds of aerial fluids, or _gases_,[63] as they are also called. Some of these, particularly that afterwards called fixed air, were discovered by Van Helmont. Considerable advances were made by a German chemist, named _Mayow_, in the last century; but his book had fallen into such oblivion that his name was scarce ever mentioned, until his discoveries were repeated, and still greater advances made by others. Dr. Hales obtained air from a great many different substances, but was unable to ascertain any thing concerning its nature. Dr. Black of Edinburgh laid the foundation of pneumatic chemistry, by discovering that a certain species of air is capable of being absorbed by earths of different kinds, and that many very heavy substances owe at least one half of their weight to this condensed air. The discovery was accidental. Wishing to obtain a very pure and white lime, he had recourse to the fine white earth called _magnesia alba_. Some of this he distilled with a heat sufficient to make the vessel red hot. Only a very small quantity of water came over, but the magnesia had lost almost two thirds of its weight. This immense loss was found to arise from an emission of air during the operation; and by other experiments it was likewise found that the air might be transferred from one portion of magnesia to another from which it had been previously expelled; that the existence of this species of air in certain bodies was the cause of that fermentation which takes place when any acid is poured upon them, as vinegar upon chalk or potash. Hence if any of these substances be deprived of its air, it will not any longer ferment in this manner. It must not be forgot, however, that when air thus unites itself with any terrestrial substance it no longer has its former properties. It is reduced exceedingly in bulk, and in proportion to this reduction only the body is increased in weight; and therefore though we say that the _air_ is absorbed, we must still remember that only _one part_ of it is so, and that by far the least considerable in bulk. A violent fire will always expel the air again, and restore it to its former bulk; and again the condensation or absorption of the air is always attended with the production of heat. This last property was not much attended to by Dr. Black, but others have observed it; and the late Dr. Charles Webster of Edinburgh published a theory in which he maintained that condensation was in _all_ cases the cause of of heat. But, however true it may be that condensation of any kind is followed or accompanied by heat, it is evidently necessary to know the cause of the _condensation_ also, otherwise we make no advance in solid theory. [63] _Gas_ is a German word, or derived from one, signifying _spirit_. The word _ghost_ comes from the same original.