DISCOVERY BY EXAMINING THE EFFECTS OF FORCES. 551 



Mariotte, that the volume of a gas at a given temperature 

 varies inversely as the degree of pressure put upon it, was 

 arrived at by subjecting gases to definite degrees of com- 

 pression. By subjecting gases to the combined influence 

 of great pressure and cold, Davy, Faraday, Natterer, and 

 others succeeded in liquefying those which could not be 

 liquefied by pressure alone. By examining polarised light 

 also whilst it was passing through bodies subjected to 

 unequal mechanical force, Brewster and others discovered 

 symmetrical changes of internal structure in bodies. The 

 entire science of photo-chemistry and the art of photo- 

 graphy were evolved by examining the effect of light upon 

 various chemical compounds. 



An immense number of chemical and other discoveries 

 have also been made by applying heat to various substances 

 and mixtures of substances. It was by this means the 

 great truth was gradually discovered, that all simple bodies 

 are converted into vapours or gases at a sufficiently 

 elevated temperature, and compound ones were either 

 similarly affected or were decomposed. The alchemists 

 employed this method extensively, and thereby discovered 

 most of the common acids, many volatile bodies, the pro- 

 perties of gunpowder, &c. Djafar, or Geber, a great 

 Arabian chemist who lived towards the end of the ninth 

 century, obtained liquid nitric acid by distilling in a 

 retort Cyprus vitriol, alum, and saltpetre; Ehazes, an 

 Arabian physician, born A.D. 860, obtained fuming sul- 

 phuric acid by distilling green vitriol ; he also prepared 

 concentrated alcohol by distilling spirit of wine with 

 quicklime. The Arabians at that period were also 

 acquainted with the effect of heat upon automatic fire, 

 made from equal parts of sulphur, saltpetre, and sulphide 

 of antimony, to which was added liquid asphaltum, a little 

 quicklime, and some juice of sycamore, and the mixture 



