344 COSMOS. 



ity of the air, which acts on combustion, on the oxydation of 

 metals, and on respiration, constituted a most powerful imped- 

 iment. 



The inflammable or light-extinguishing gases occurring in 

 caverns and mines (the sjnritus letales of Pliny), and the es- 

 cape of these gases in the form of vesicles in morasses and 

 mineral springs, had already attracted the attention of Basilius 

 Valentinus, a Benedictine monk of Erfurt (probably at the 

 close of the fifteenth century), and of Libavius, an admirer 

 of Paracelsus, in 1612. Men drew comparisons between that 

 which was accidentally observed in alchemistical laboratories^ 

 and that which was found prepared in the great laboratories 

 of nature, especially in the interior of the Earth. The work- 

 ing of mines in strata, rich in ores (especially those containing 

 iron pyrites, which become heated by oxydation and contact- 

 electricity), led to conjectures of the chemical relation existing 

 between metals, acids, and the external air having access to 

 them. Even Paracelsus, whose visionary- fancies belong to 

 the period of the first discovery of America, had remarked the 

 evolution of gas when iron was dissolved in sulphuric acid. 

 Van Helmont, who first employed the term gas, distinguished 

 it from atmospheric air, and also, by its non-condensibility, 

 from vapors. According to him, the clouds are vapors, and 

 become converted into gas, when the sky is very clear, " by 

 means of cold and the influence of the stars." Gas can only 

 become water after it has been again converted into vapor. 

 Such were the views entertained in the first half of the sev- 

 enteenth century regarding the meteorological process. Van 

 Helmont was not acquainted with the simple method of tak- 

 ing up and separating- his gas sylvestre (the name under which 

 he comprehended all uninflammable gases which do not main- 

 tain combustion and respiration, and differ from pure atmos 

 pheric air) ; but he caused a light to burn in a vessel under 

 water, and observed that, when the flame was extinguished, 

 the water entered, and the 'volume of air diminished. Van 

 Helmont likewise endeavored to show by determinations of 

 weight (which we find already given by Cardanus) that all 

 the solid portions of plants are formed from water. 



The alchemistic opinions of the Middle Ages regarding the 

 composition of metals, and the loss of their brilliancy by com- 

 bustion in the open air (incineration, calcination), led to a de- 

 eire of investigating the conditions by which this process was 

 attended, and the changes experienced by the calcined metals, 

 and by the air in contact with them. Cardanus, as early as 



