342 EPOCHS IN THE HISTOBY OF THE CONTEMPLATION OF 



But if, as we have already remarked, by the invention of 

 appropriate although still very imperfect physical instru- 

 ments, and by the sagacity of Galileo, Torricelli, and the 

 members of the Accademia del Cimento, the relations of 

 temperature, the variations of the atmospheric pressure, and 

 the quantity of vapour in the air, became objects of imme- 

 diate research; on the other hand, all that regards the 

 chemical composition of the atmosphere remained wrapped 

 in obscurity. The foundations of (< pneumatic chemistry" 

 were indeed laid by Johann Baptist van Helmont and Jean 

 Eey, in the first half, and by Hooke, Mayow, Boyle, and 

 the dogmatising Becher in the latter half of the 17th 

 century ; but however striking was the correct apprehension 

 of particular and important phenomena, yet the insight into 

 their connection was wanting. The old belief in the 

 elementary simplicity of the air which acts in combustion, 

 in the oxydation of metals, and in respiration, formed an 

 obstacle difficult to be overcome. 



The inflammable or light-extinguishing kinds of gas oc- 

 curring in caves and mines (the " spiritus letales" of Pliny), 

 and the escape of these gases in the shape of bubbles in marshes 

 and mineral springs, had already arrested the attention of the 

 Erfurt Benedictine monk Basilius Valentinus, who probably 

 belonged to the close of the 15th century, and of Libavius, 

 an admirer of Paiacelsus, in 1612. Comparisons were drawn 

 between what was accidentally remarked in alchemistic labo- 

 ratories, and what was seen to have been prepared in the great 

 laboratories of nature, especially in the interior of the earth. 

 Mining operations in beds rich in ore, (particularly such as 

 contained pyrites which become heated by oxydation and 

 contact electricity), led to anticipations of the chemical 



