342 



HISTORY OF SCIENCE. 



It was Hales who first elucidated the functions of the sap. But his 

 name will be ever remembered in chemistry as the inventor of the 

 Pneumatic Trough, an apparatus simple enough, but of the greatest 

 importance to the progress of chemistry. As we have already seen, a 

 few experimenters had succeeded in collecting a "certain kind of air" 

 (hydrogen) by inverting a bottle filled with dilute acid over pieces of 

 iron immersed in an open vessel of the same liquid. By Hales's 

 arrangement the gas generated in one vessel could be collected in 

 another, and the power which this gave to the chemist of treating the 

 materials in his generating-vessels in any manner he thought fit, will 



be at once understood from the 

 woodcut representing the appa- 

 ratus, which is copied in Fig. 

 169, and renders a detailed de- 

 scription unnecessary. The tube 

 of the retort on the furnace is 

 turned up so as to pass loosely 

 into the neck of a bottle sus- 

 pended mouth downwards, and 

 completely filled at the begin- 

 ning with water, which it retains 

 in its inverted position in con- 

 sequence of its mouth dipping 

 below the surface of the water 

 in the pail. The tube leading 

 the gas from the place where it 

 is generated to the mouth of the 

 inverted vessel is therefore the 

 special feature of Hales' invention. The " pneumatic trough " used 

 at the present day differs from Hales' apparatus only in having a more 

 convenient arrangement of its parts. 



Though Hales invented the most convenient apparatus in which 

 gases can be collected, he does not seem to have himself discovered 

 and recognized any gas as such. Yet he actually collected gases re- 

 sulting from the distillation of wax, etc. gases which would not be 

 very different from common illuminating coal-gas and he found that 

 these were inflammable. Unfortunately, a preconceived idea blinded 

 him to the real facts which his experiments disclosed. He considered 

 the inflammable gas to be air, owing its inflammability only to the 

 sulphurous and oily vapours with which it was impregnated. It was 

 a settled theory with Hales that atmospheric air was the elementary 

 principle by which the particles of all bodies were bound together. 



Many of Hales' experiments were repeated by BOERHAAVE (1688 

 1738), the celebrated Dutch physiologist, with whom chemistry was 

 a favourite study. He was one of the first to treat the question of the 

 manure applied to land as one based upon scientific principles. He 



FIG. 169. 



