728 COSMOS. 



perature, the alternations in the pressure of the atmosphere, 

 and the quantity of the vapour contained in it, were made 

 made the object of direct investigation by means of the inven- 

 tion of appropriate, although still very imperfect, physical in- 

 struments, and by the acute penetration of Galileo, Torricelli, 

 and the members of the Academia del Cimento ; all that refers 

 to the chemical composition of the atmosphere, remained, on 

 the other hand, shrouded in obscurity. The foundations of 

 pneumatic chemistry were, it is true, laid by Johann Baptist 

 von Helmont, and Jean Hey, in the first half of the seventeenth 

 century, and by Hooke, Mayow, Boyle, and the dogmatising 

 Becher in the closing part of the same century, but however 

 striking may have been the correct apprehension of detached 

 and important phenomena, the insight into their connection 

 was still wanting. The old belief in the elementary simplicity 

 of the air which acts on combustion, on the oxidation of metals 

 and on respiration, constituted a most powerful impediment. 



The inflammable, or light-extinguishing gases occurring in 

 caverns and mines (the spiritus letales of Pliny), and the escape 

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

 springs, had already attracted the attention of Basilius Valen- 

 tinus, a Benedictine monk of Erfurt, (probably at the close of 

 the fifteenth century) and of Libavius, an admirer of Paracel- 

 sus, 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 working of mines 

 in strata, rich in ores, (especially those containing iron pyrites, 

 which become heated by oxidation 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- condensability, 

 from vapours. According to him the clouds are vapours, 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 vapour. 

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



