138 GREAT MEN OF SCIENCE 



weighed anything - anything positive, or even negative. It 

 seemed in some way to be a constituent of the combustible 

 body, a constituent which became free upon burning, per- 

 haps identical with the fire-stuff, phlogiston, which had been 

 imagined for some eighty years as passing from one body 

 to another in the form of flame, when burning takes place. 

 But after the weighings carried out since Black's time, and 

 even by Boyle, difficulties had arisen with respect to weight 

 in the case of heat, phlogiston and light, even if one had been 

 willing to regard the invariability of the weight of all things 

 concerned in the reaction, as assured in the case of these 

 mysterious phenomena of fire. For this it would have been 

 necessary to take up a strongly materialistic attitude, re- 

 garding what cannot be weighed as of no importance, and 

 this was just as little characteristic of these three investi- 

 gators as of Kepler or Newton. This alone perhaps explains 

 the manner in which they all held firmly to the concept of 

 phlogiston,^ which was even defended energetically by 

 Priestley, 2 but it was just these three investigators, who, 

 as Boyle and Black had already done, did everything towards 

 learning by the only possible means - namely common 

 observation of nature - how matters really stand as regards 

 weighable or unweighable stuff taking part in chemical 

 transformations. With the addition of their investigations, 

 just enough facts were known for an attempt to be made 

 with some certainty to discover whether we may reckon with 

 invariability in the weights of the substances concerned in a 



^ Perhaps this explanation does not hold in Scheele's case. He says in 

 his Chemical Essays on Air and Fire (1777): 'that light must be regarded 

 like heat, as a substance, cannot be doubted.' According to this, he 

 cannot have been well acquainted with Guericke and Huygens. Alone 

 the multiplicity of the things considered as of uncertain weight - heat, 

 light, phlogiston, - must have much increased the difficulty of arriving 

 at more correct views. 



2 May it not be that Priestley was also influenced by a justifiable dis- 

 inclination to accept, before exhaustive proofs had been given, the ideas 

 of a man who, like Lavoisier, had made so many incorrect statements 

 with regard to facts relating to the origin of discoveries.' 



