MODERN SCIENCE. 175 



oxide of mercury, by means of a burning glass. He called it 

 " DEPHLOGISTICATED AIR " the name of oxygen being 

 given to it by Lavoisier, who understood its property and 

 character. The sterility of this great discovery in Priestley's 

 hands is due partly to his blind adherence to StahPs phlogistic 

 theory, and partly to the carelessness of his experiments. 

 Priestley did great service to science, nevertheless. He 

 determined the properties of NITROUS AIR (nitric acid), 

 vitriolic acid air (sulphuric dioxide), muriatic acid air (hydro- 

 chloric acid), and alkaline air (ammonia). His experiments 

 on inflammable air (hydrogen) drew the attention of chemists 

 to this gas, and led them to numerous results. He discovered 

 that PLANTS INHALE the vitiating part (say fixed air) which 

 the atmosphere may contain, and thereby restore air to its 

 purity. This was one more step, after Grew and Hales,, 

 towards the explanation of the breathing of plants, but he 

 failed to see the reason why. It is now known that plants 

 absorb carbonic acid, decompose it in order to live on the 

 carbon, which they retain, whilst they reject the oxygen. 

 Priestley assisted MANIPULATION by several inventions the 

 most serviceable of which is the pneumatic trough. 



1735 84. Bergmann studied the theory of elective or 

 CHEMICAL AFFINITIES. Newton had shown the powerful 

 attraction of some substances with one another, thus taking 

 the first step in the study which Bergmann pursued. This 

 great chemist drew up a table of those substances which show 

 affinity with one another (called Table of Elective Affinities). 

 The French chemist Geoffroy had drawn up Tables of 

 Affinity (the term is his own) as early as 1718, indicating the 

 order in which bodies displace each other, and thus marking 

 to some extent the force of affinity. The word as used in 

 chemistry means " a disposition to unite." Bergmann's 

 work extended much further than this study. During his 

 experiments to establish his tables, he found out Black's fixed 

 air to be an acid which he called air acid or AERIAL ACID ; 

 but he failed to find out that it is a compound of two elements. 

 It was Lavoisier who discovered it to be made of carbon and 

 oxygen, and gave it the name of carbonic acid which de- 

 scribes its character at once. Next to this, Bergmann founded 



