1815.] Dr. Joseph Black. 325 



comforts of the numerous descendants of his worthy parents — heirs, 

 not only of their name, but likewise of their unambitious modera- 

 tion and amiable simplicity of character." 



The publications of Dr. Black (omitting his inaugural disserta- 

 tion) amount only to three short papers; for respecting his discove- 

 ries and speculations concerning heat, he published no account at 

 all. In the year 1803, or three years after his death, his lectures 

 were published by Dr. John Kobisun, Professor of Natural \ hilo- 

 -«ophy in the University of Edinburgh, who undertook the task of 

 editor at tlie request of Dr. Black's friends. They appear to me to 

 be a pretty fair representation of his lectures such as they were 

 about the year 1796 ; and though it was by no means doing justice 

 to Dr. Black to give to the world lectures never intended for publi- 

 cation, and just at a period too when the science had assumed a 

 new form, quite different from that under which he had been accus- 

 tomed to view it ; yet they convey a very happy view of the eleaant 

 simplicity of manner by which he was distinguished, of the very 

 apt illustrations by which the different subjects that he treated were 

 elucidated, and of the true philosophical caution with which all iiis 

 conclusions were drawn. 



His Experiments upon Magnesia Alba, Quick-Lime, and other 

 Alkaline Substances, were published in 17^5, and constitute in my 

 opinion one of the very best specimens of analytical investigation 

 ever offered to the public. It was known that when lime-stone is 

 exposed to a violent heat for a sufficient time, it is converted into 

 quick-lime, a substance which has a strong caustic taste, gives a 

 green colour to vegetable blues, corrodes animal and vegetable sub- 

 stances, &c. If pearl ash be mixed with three or four times its 

 weight of quick-lime, and agitated for some hours in a quantity of 

 water, the quick-lime is converted ii\to chalk, and the pearl-ash 

 becomes exceedingly caustic and corrosive. These facts were suffi- 

 ciently known ; but no satisfactory explanation of them had been 

 offered, though it was generally believed that the fire had imparted 

 something to the lime to which it was indebted for its causticity. 

 Dr. Black demonstrated, by the most decisive experiments, that 

 lime-stone is a compound of two substances ; namely, quick-lime 

 «nd a peculiar species of air which acts the part of an acid, and to 

 which he gave the name of Jixcd air. The fire disengages this air, 

 and drives it oft". The quick-lime remains in a state of purity, and 

 exhibits its natural caustic properties. Pearl-ash is in like manner 

 a compound of pure potaih and fixed air. Quick-lime has a stronger 

 affinity for fixed air than potash has. Accordingly it separntes the 

 fixed air from the alkali, and is converted into lime-stone; while 

 the alkali, thus freed from the acid with which it was united, ex- 

 hibits its natural caustic properties in all their energy. Such is the 

 outline of the theory which Dr. Black establishes in his dissertation. 

 He cstahlixhes likewise the peculiar properties of magnesia, and 

 •hows that it differs from every other earthy body. 



