J 9 8 SCIENTIFIC APPARATUS. 



therefore supposed that the phlogiston had left the charcoal to unite 

 with the calx of lead forming the original metal. They found that 

 almost any combustible was capable of effecting this change, and 

 they employed for the purpose other metals, iron and zinc, and 

 other combustibles, sugar, coal, and flour. The process being a 

 general one, it was natural to suppose that the phenomenon was 

 due to the transference of the phlogiston from one body to the 

 other. Boyle had previously shown that during the calcination of 

 tin an increase of weight takes place, which seems at variance with 

 the supposed loss of phlogiston in the process ; the phlogistiaps, 

 however, were equal to the occasion, and ascribed to their subtle 

 principle a property of levity, which explained the iact. They 

 considered phlogiston as a dry substance of an earthy nature, 

 since most combustibles are insoluble in water, a property which 

 was supposed to be common to all earths. The more enlightened 

 followers of this school did not, however, regard the separate exist- 

 ence of phlogiston as necessary : for Bishop Watson, of Llandaff, 

 wrote that a " handful of phlogiston was not to be expected." 



The researches on gases towards the end of the last century 

 exerted a great influence on the theory of chemistry. The term 

 gas was first employed by Van Helmont at the beginning of the 

 previous century ; but although he seems to have suspected the 

 existence of different, kinds of gas, he confused several under the 

 name of gas sylvestre, which he says will not support combustion ; 

 he states that it is evolved during the fermentation of wine and beer, 

 that it is produced when charcoal is burnt in air, and when marble or 

 chalk is dissolved in distilled vinegar : in these processes we know 

 that carbonic acid gas is formed. But he also gives the name of gas 

 sylvestre to products from other operations which are now known 

 to form the oxides of nitrogen and other gases. About a hundred 

 years later, Dr. Hales published a treatise on various kinds of 

 air; and afterwards the study of pneumatic chemistry was much 

 advanced by the experiments of Black, Cavendish, and Priestley. 

 ^Black showed that the difference between the caustic and mild 



