258 A SHORT HISTORY OF SCIENCE 



spheres he showed that eight horses on either side were unable to 

 overcome this pressure on a particular globe which he had con- 

 structed and exhausted of air. These various experiments and dis- 

 coveries relating to atmospheric pressure led to the investigations 

 and laws of Boyle, Mariotte, and others and, less than a century 

 later, to the steam-engine of Watt, in which steam was at first 

 used only to produce a vacuum, atmospheric pressure being 

 employed as the moving force. 



FURTHER STUDIES OF THE ATMOSPHERE : GASES. Meantime 

 the chemical composition of the atmosphere was being no less 

 eagerly studied. Robert Boyle (1627-1691) published at Oxford 

 in 1660, New Experiments Physico-Mechanical touching the 

 Spring of the Air and its Effects, and in his Sceptical Chymist 

 gives an interesting and instructive picture of the chemical ideas 

 of his time. He was the first to insist on the difference between 

 compounds and mixtures, and probably the first to use the pneu- 

 matic trough for the collection and study of gases. 



The word "gas" was introduced by Van Helmont (1577-1644), 

 who by virtue of the following remarkable statement deserves to 

 be remembered as the principal chemist of the earlier half of the 

 seventeenth century : 



Charcoal and in general those bodies which are not immediately re- 

 solved into water, disengage by combustion spiritum sylvestrum. From 

 62 Ibs. of oak charcoal 1 Ib. of ash is obtained, therefore the remaining 

 61 Ibs. are this spiritum syhestre. This spirit, hitherto unknown, I 

 call by the new name of gas. It cannot be enclosed in vessels or re- 

 duced to a visible condition. There are bodies which contain this 

 spirit and resolve themselves entirely into it : in these it exists in a 

 fixed or solidified form, from which it is expelled by fermentation, as we 

 observe in wine, bread, etc. 



It has been well said that 



this passage is remarkable not only for the explicit mention of car- 

 bonic acid gas (as we now call it) as a product of fermentation, and for 

 the introduction of the word gas for the first time, but also for its ap- 

 peal to the balance, 



