The Great Chemist, Joseph Black. 55 



hypothesis was framed to account for this. The idea of an 

 imponderable in Chemistry was common enough, but for the 

 then purpose the facts required that the weight of the Phlogiston, 

 or whatever name the caustic principle might be called, should 

 be negative, surely something so conti-ary to the usual expeiience 

 of matter that it might have given rise to more than doubt in the 

 minds of the observers. 



At first Black held the common view. He seems to have 

 thought of the possibility of catching this caustic substance as it 

 escaped into the air while the caustic lime gradually became 

 transformed to calcareous earth. In his note books a couple of 

 years before his degree thesis, experiments are mentioned with 

 this object. He placed some caustic lime in a saucer floating in 

 water and inverted a glass vessel over it. He notes the fact that 

 nothing escapes, not only does the water not fall in level by 

 increase of occupied space in the cover glass, but it actually rises 

 slightly. Thus early is he upon the track of the solution. I 

 need not follow his experiments in detail, but he succeeded in 

 proving conclusively that causticity was due to the loss of a 

 constituent in the gaseous form, which he named " fixed air." 

 He showed the transfer of this fixed air from one alkali to 

 another. He weighed and measured approximately the mass and 

 volume of the fixed air, and showed that it maintained an 

 unalterable relation to the mass of the particular alkali employed. 

 He inferred its possible identity with the iixed air obtained by 

 combustion of vegetable matter, and with a constituent of the 

 air expired from the lungs of living beings. In fact he really and 

 truly discovered carbonic acid, even to the extent of teaching 

 that its compounds with the alkalies were exactly analogous to 

 those formed by the mineral acids. This was the first time it 

 had dawned upon chemists that a substance which when alone 

 existed only in the gaseous form, might in combination be a solid 

 and appear as a stone. The advance in chemical knowledge was 

 inconceivable, and it is no wonder that he is looked upon as the 

 father of Pneumatic Chemistry, or that the great Lavoisier should 



