CHEMISTRY OF THE EIGHTEENTH CENTURY. 347 



thecary, while constrained to admit Black's facts, opposes his own theory 

 to that of the eminent Scotchman, and argues in its favour with suffi- 

 cient plausibility to find adherents. According to him, if limestone is 

 rendered caustic by heat, it is not because it has parted with "fixed 

 air" but because it has absorbed from the fire a particular kind of 

 acid, which Meyer calls acidiun pingue. It might be objected to this 

 theory that the limestone loses weight in the fire instead of gaining it. 

 The theorist is ready with the reply that acidum pingue is a substance 

 similar to heat and light, or that it is anti-gravitating, etc. But it may 

 be objected to the theory that acidum pingue cannot be produced arid 

 proved to have a real and independent existence. No one can for a 

 moment doubt of the validity of this objection in determining his choice 

 between Black's and Meyer's theories ; yet acidum pingue is not the 

 only imaginary existence to which theoretical plausibility lias given 

 acceptance. Caloric, elective affinity, vital force, the luminiferous 

 ether, and many other very respectable theoretical entities id genus 

 omne are liable to the same objection. But acidum pingue has fallen 

 into merited oblivion. It may be interesting to see, by a quotation 

 from a paper presented to the Academy of Sciences in 1764, how 

 troublesome facts are when they do not fit the theories. The writer 

 shows that when lead is converted into litharge (oxide of lead\ a real 

 increase of weight takes place, amounting to about one-eighth. " This 

 is," he remarks, " truly a paradox, though experiment puts it beyond 

 all doubt. But though it is easy to establish the fact, it is not so easy 

 to give a satisfactory explanation of it ; it eludes all the physical notions 

 which we possess, and only from time can the solution of this difficulty 

 be looked for." The resolution to wait for the knowledge that time 

 might bring was wiser than attempting to warp the facts ; but had the 

 chemist (Tillet), who so clearly saw the incongruity of the fact with the 

 phlogiston theory, had but the courage to relinquish this theory alto- 

 gether and seek for a better one, he might have gathered some of the 

 laurels which fell to the lot of Lavoisier a few years later. 



Among the English chemists who continued the investigation into 

 the nature of " fixed air " (carbonic acid gas), none surpassed in the 

 precision and success of their researches the celebrated HENRY CAVEN- 

 DISH (1731 1810). He was the son of Lord Charles Cavendish, the 

 brother of the third Duke of Devonshire. Lord Charles Cavendish 

 died in 1783, when he was the senior member of the Royal Society, 

 which he had entered in 1727. He was a good mathematician, and 

 made some barometrical determinations that were considered of value. 

 His son inherited these scientific tastes, manifesting at an early age a 

 decided taste for scientific pursuits ; to which, indeed, he ultimately 

 devoted himself with ardour, sacrificing every ordinary ambition, and 

 contenting himself with a very moderate patrimony. Unexpectedly he 

 came into the possession of a large fortune after he was forty years of 

 age. He became a member of the Royal Society in 1760, and for a 



