BLACK. 



551 



Jlaek. the object of the second part of this important essay, 

 -v ' to ascertain the difference between quick-lime and 

 limestone, mild >nd caustic alkalies, common and 

 calcined magnesia. By the most simple but decisive 

 experiments he demonstrated, that limestone and the 

 mild alkalies contain a quantity of air fixed in them ; 

 that this air makes its escape when they are dissolved 

 in acids; that it is dissipated when limestone is burnt; 

 that alkalies are rendered caustic by being deprived of 

 it ; that lime has a greater affinity for it than alkalies ; 

 that it takes it from them when mixed with them. 

 Hence the reason why quick-lime renders alkalies 

 caustic, while, by the process, it is converted itself 

 into limestone. The caustic alkalies, quick-lime, 

 and calcined magnesia, are the substances in a pure 

 tate ; hence the reason of the activity which they 

 possess, and of their dissolving in acids without effer- 

 vescence. This air he called jixed nir, because it 

 exists fixed in their bodies. He inferred, that it pos- 

 sessed peculiar properties ; that it differed from com- 

 mon air ; that it existed in the atmosphere ; and that 

 there are different kinds of air possessed of peculiar 

 properties. But he made no experiments himself on 

 the subject, but drew his inferences from common 

 observations ; of a kind, however, sufficiently deci- 

 sive. He even shewed, that it acted as an acid; that 

 it combined with alkaline substances in different pro- 

 portions ; and he announced his intention of prose- 

 cuting the subject at greater length hereafter ; a pro- 

 mise, however, which he never fulfilled. It is impos- 

 sible to bestow too high praise upon Dr Black's pa- 

 per on magnesia and quick-lime. The modest sim- 

 plicity of the narration, the number and conclusive- 

 ness of the experiments, the sagacity with which the 

 proper inierences are drawn, the accuracy and deci- 

 siveness of the reasoning, and the small number of 

 mistakes into which the author fell in prosecuting a 

 subject entirely new, and quite the opposite of the 

 preconceived opinions of chemists, are all admirable, 

 and account sufficiently for the high rank to which 

 they immediately raised the author among chemists. 

 Indeed, we know of no chemical dissertation what- 

 ever that will stand a comparison with the essay of 

 Dr Black. Mr Lavoisier's papers are much more 

 elaborate, at least in appearance, and the consequences 

 which he deduces from his experiments are, perhaps, 

 of more importance than even the discoveries of Dr 

 Black ; but his discoveries were the result of a whole 

 life, spent in the most laborious industry, whereas Dr 

 Black's great discovery was completed at the age of 

 twenty-five. 



Soon after the publication of Dr Black's essay, 

 Mr Meyer, an apothecary at Osnaburgh, published 

 a dissertation, in order to explain the differences be- 

 tween limestone and quicklime, the caustic and mild 

 alkalies. Hia conclusions were quite different from 

 those of Dr Black. According to him, limestone 

 combines in the fire with a peculiar acid, which he 

 called the acidum pinguc, or causlkunu To this 

 acid it owed the peculiar acrid properties which it 

 acquired by calcination. Alkalies had a stronger af- 

 finity tor this acid than lime. Hence, when potash 

 and quicklime are mixed, the lime loses its acid, and 

 becomes tasteless and insoluble in water ; while the 

 alkali unites with the acid and becomes caustic. 



Meyer's hypothesis being contrasted with Dr Black's 

 theory, which soon became known in Germany, oc- 

 casioned a violent controversy, which lasted some 

 years. Jacquin, botanical professor at Vienna, pub- 

 lished a Latin dissertation in defence of Dr Black's 

 doctrine in 1769. This was opposed, in 1770, by 

 Dr Crans, physician to the king of Prussia, who 

 defended the hypothesis of Meyer in a very elaborate 

 treatise. In 1774, Lavoisier published a treatise on 

 the subject. He repeated the experiments of Dr 

 Black and his disciples, and confirmed them in every 

 particular. Since that period, Dr Black's theory 

 of causticity, and his doctrines respecting fixed air, 

 have been universally admitted by the chemical pub- 

 lic. 



Dr Black's essay drew the attention of philoso- 

 phers to the elastic fluids, and in particular to Jixed 

 air, which he had shewn to act so important a part 

 in nature. Dr Macbride of Dublin was the first 

 who wrote on the subject. His essay appeared in 

 176i. He examined the fixed air evolved during 

 fermentation, and pointed out its properties in re- 

 tarding putrefaction. But it was Mr Cavendish who 

 first examined the properties, and pointed out the 

 characters of fixed air. His essays on the subject 

 were published in 1766 and 1767. Dr Priestley fol- 

 lowed soon after, and laid open the different elastic 

 fluids in succession. Thus Dr Black's discovery is 

 not only important in itself, but it acquires addition- 

 al value, because it led the way to pneumatic che- 

 mistry, and was therefore the foundation of the 

 complete revolution which the science underwent. 



2. The paper on the freezing of boiled water, 

 published in the Philosophical Transactions, is very- 

 short, and requires but little notice. He found, 

 that water which has been newly boiled always 

 freezes sooner than common water. The reason, he 

 says, is, that boiled water cannot be cooled lower than 

 32, without beginning to freeze ; whereas commori 

 water may be cooled several degrees lower, without 

 losing its fluidity. This difference he ascribes to the 

 boiling, which has deprived this water of its air. 

 Hence, when exposed to the air, that elastic fluid 

 begins to enter, and occasions a constant agitation, 

 which, though imperceptible to us, is yet sufficient 

 to prevent the water from being cooled lower than 

 32 without freezing. This explanation is simple 

 and ingenious, and probably correct. Though some 

 of the experiments of Sir Charles Blagden, on the 

 cooling of water below the freezing point, without 

 losing its fluidity, appear at first sight incompatible 

 with it. 



3. The analysis of the Geyser and Reykum waters, 

 which Dr Black published in the second volume of 

 the Edinburgh Transactions, is one of the finest 

 specimens of the analysis of mineral waters which ' 

 has yet appeared. Dr Black has contrived to throw 

 an interest on his subject, of which one would hardly 

 have conceived. that it would have admitted. This 

 appears in a very striking point of view, when com- 

 pared with the essays of Bergman, Klaproth, Four- 

 croy, Vauquelin, or any other of the consummate 

 masters of the art of analysing mineral waters. The 

 interest which we take in the analysis of Dr Black, 

 is much greater than in that of any other person, The 



Black. 



