8 HISTORY OF CHEMISTRY. [LECTURE I. 



phlogiston. It was a new application of the element fire. 

 This double principle the combustible principle on one hand, 

 the ponderable fire material on the other satisfactorily and 

 completely explained the phenomena of combustion to the 

 chemists at the close of the seventeenth century. We find 

 these views first shaken by Newton, for whom fire is not a 

 special substance. He suggests that every strongly heated and 

 glowing substance burns; that red-hot iron or wood may be 

 called fire ; and that those substances which emit much smoke 

 burn with a flame. 



The assumption of this ponderable fire material was first 

 recognised as really fallacious in consequence of a highly in- 

 teresting experiment by Boerhave, who weighed masses of 

 metal both cold and red-hot and found their weights to be 

 identical in both cases. 7 The explanation of the increase of 

 weight, next brings about differences of views amongst the 

 chemists of the eighteenth century. Some seek to regard it, as 

 Stahl had done, as an unimportant phenomenon which may be 

 neglected ; others, on the contrary, and amongst them Boer- 

 have, assume a union with certain (saline) portions of the air, 

 and in this way seek to take account, at the same time, of the 

 necessary presence of air during the combustion and of the 

 increase in weight. According to others again the air merely 

 serves to take up the separated phlogiston, which, in their 

 view, cannot escape from one substance if there is not another 

 present with which it can unite. In the middle of the 

 eighteenth century we also find the notion that phlogiston 

 possesses negative weight, or absolute levity. It seems quite 

 natural to the upholders of this hypothesis that the weight in- 

 creases on the separation of phlogiston. Others still, who 

 have difficulty with the conception of absolute levity, regard 

 phlogiston as lighter than air. This view is upheld, for 

 example, by Guy ton de Morveau, 8 whose explanation of the 

 increase of weight is based upon the Archimedean principle, 

 and does not altogether tell in favour of the clearness of imagi- 



7 Kopp, Geschichte. 3, 127. 8 Ibid. 3, 149. 



