THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 343 



relations Between metals,, acids, and the air which gained 

 access from without. Paracelsus, whose fancies belong to 

 the epoch of the first conquests in America, already remarked 

 the disengagement of gas when iron was dissolved in sul- 

 phuric acid. Yan Helinont, who first made use of the 

 word " gas," distinguishes gases from atmospheric air, and 

 also, on account of their non-condensability, from vapours. 

 He regards the clouds z& vapours, which, when the sky is 

 very clear, are changed into gas "by cold and by the 

 influence of the heavenly bodies." Gas, he says, can only 

 become water when it has previously been retransformed 

 into vapour. These views of meteorological processes be- 

 longed to the first half of the 17th century. Yan Helmont 

 was not yet acquainted with the simple means of receiving 

 and separating his " Gas sylvestre," (under which name he 

 included all uninflammable gases different from pure atmo- 

 spheric air, and incapable of supporting flame and respira- 

 tion) ; yet he made a light burn in a vessel having its mouth 

 in water, and remarked that as the flame went out, the water 

 entered, and the " volume of air" diminished. Yan Hel- 

 mont also sought to demonstrate by determinations of weight, 

 (which we find already in Cardanus), that all the solid parts 

 of plants are formed from water. 



The mediaeval alchemistic opinions of the composition of 

 metals, and of their combustion in air whereby their bril- 

 liancy was destroyed, incited to the examination of what took 

 place during the process, and of the changes undergone by 

 the metals themselves, and by the air in contact with them. 

 Cardanus had already become aware in 1553 of the increase 

 of weight that takes place during the oxidation of lead, and, 

 quite in the spirit of the phlogistic hypothesis, had ascribed 



