text: Stephen Hales, called the creator of pneumatic chemistry, created the pneumatic trough in 1727. This instrument was widely used by many chemists to explore the properties of different airs, such as what was called inflammable air (what is modernly called hydrogen). Lavoisier used this in addition to his gasometer to collect gases and analyze them, aiding him in creating his list of simple substances.
  The pneumatic trough, while integral throughout the eighteenth century, was modified several times to collect gases more efficiently or just to collect more gas. For example, Cavendish noted that the amount of fixed air that was given off by a reaction was not entirely present above the water; this meant that fixed water was absorbing some of this air, and could not be used quantitatively to collect that particular air. So, he replaced the water in the trough with mercury instead, in which most airs were not soluble. By doing so, he could not only collect all airs given off by a reaction, but he could also determine the solubility of airs in water, beginning a new area of research for pneumatic chemists. While this was the major adaptation of the trough in the eighteenth century, several minor changes were made before and after this substitution of mercury for water, such as adding a shelf to rest the head on while gas collection occurred. This shelf would also allow for less conventional heads to be used, such as Brownrigg's animal bladder.
  A practical application of a pneumatic trough was the eudiometer, which was used by Jan Ingenhousz to show that plants produced dephlogisticated air when exposed to sunlight, a process now called photosynthesis.