JOSEPH BLACK 129 



he also brought forward matters of fundamental importance, 

 namely the question of the mutual relationship of 'mild' and 

 'caustic' alkalis, as for example, chalk and lime. It was 

 supposed that chalk when ignited gave off 'fire-stuff' (which 

 as 'phlogiston' played a great but somewhat mysterious part 

 in the thought of that period), and that it thereby became the 

 corrosive 'quicklime.' Black showed by thorough experi- 

 ments that it does not take up anything that is perceptible, 

 but on the contrary, gives up something when ignited, namely 

 a kind of air; the same, namely that also results from 

 fermentation (to-day called carbonic acid gas). He also 

 proved that this production of gas is associated with a loss of 

 weight. He recognised this kind of air quite generally as an 

 essential component of all mild alkalis (such as unburnt 

 magnesia, potash, soda), in which it is fixed in a solid form, 

 and for this reason he called it 'fixed air.' 



This was the first case in which a gas was recognised with 

 certainty as a weighable constituent of a solid body. Also, 

 for the first time the study of gaseous bodies was shown to 

 be of importance. All sorts of varieties of 'air' had appeared 

 in chemical experiments,but they were merely looked upon as 

 ordinary air with something mixed with it, and not as special 

 substances. At the same time, the idea of phlogiston, or 

 fire-stuff, in the sense of a constituent of chemical com- 

 pounds, became for the first time rightly suspect. Scheele, 

 Priestley and Cavendish, and as a result of their work, 

 Lavoisier, followed the matter up even during Black's life- 

 time, while Black himself approached the question of fire 

 in quite a different manner, by means of the calorimetric 

 investigations of which we have spoken. 



Joseph Black was born in the south of France, but his 

 family came from Scotland; he was one of thirteen children. 

 His education and first training were received in Ireland; 

 from the age of eighteen he studied medicine and natural 

 science at the University of Glasgow, and four years later in 



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