If we were to estimate the merits of a literary man from•the bulk of his writings, Dr Black would be rated very low. The only pieces which he published in his lifetime were four essays : 1st, His inaugural dissertation, entitled, De .aeido a cibis orto et .k mag via; 2d,_Eaperiments upon magnesia alba, quick lime, and other alkalire substances, first published in the Edinburgh Physical and Literary Essays in 1755, and afterwards in 3 separate form by Mr Creech ; 3d, Observations on the more readyfreezing c water that has been boiled, published in the Philosophical Trans actions of London for 177.1. ; and, 4th, Analysis of the waters of some boiling springs in Iceland, publish. ed in the second volume of the Edinburgh Transac tions. After his death, his manuscript lectures were revised by Professor Robison, and published by his executors in two quarto volumes. This book pos sesses great merit, especially the first part of it which treats of Heat. The simplicity of the style, and the exquisite taste and propriety displayed in the illus trations, cannot be too highly praised ; though, as a whole, it is certainly very different from what it would have ben, had it received the last corrections of the author himself. The arrangement in many parts is extremely defective ; and, as a collection of chemical facts, it is nearly twenty years behind the period at which it finally appeared. Had it been published in 1788, it would have contributed essentially to pro mote the progress of the science ; it would have in creased the reputation of its author, and been a high ly popular book even in the hands of the students ; but, in 1803, its place was supplied by others, exh; biting a fuller detail of chemical facts, and bringing the progress of the science down to the date of their publication.
In taking a view of these different publications, we may pass over the first entirely. The second was, in fact, a repetition, or rather a more complete inves tigation of the different points touched upon in the in augural dissertation.
1. The essay on magnesia and quick-lime embraces two distinct objects: 1st, The properties of magne sia ; 2d, An investigation of what constitutes the difference between quick•lime and limestone, between the mild and caustic alkalies.
Magnesia had been discovered by a Roman canon, at Rome, about the beginning of the last century, and a few of its properties had been ascertained by Frederick Hoffmann ; but chemists, in general, con sidered it as a variety of lime, and none knew the pe culiar characters by which it is distinguished. Dr Black ascertained its properties with precision, and de monstrated that it is a peculiar earth. To him, there fore, in reality, we are indebted for our knowledge of magnesia. He combined magnesia with sulphuric, nitric, muriatic, and acetic acids, and ascertained the properties of the salts formed ; compared them with similar salts of lime, and pointed out the essential differences. He determined the effect of heat upon magnesia ; shewcd that it deprived it of a•quantity of air ; that the magnesia continued still tasteless and insoluble in water ; that it combines with acids with out effervescence, and forms with them the very same salt as common magnesia. He determined the
affinity of magnesia for acids, when compared with lime and alkalies ; and pointed out the difference be tween it and alumina, and the earth of bones.
The investigation and determination of the proper. ties of magnesia, though an object of great import ance, sinks into insignificance when compared with the object of the second part of this important essay, to ascertain the difference between quick-lime and limestone, mild 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 state ; hence the reason of the activity which they possess, and of their dissolving in acids without effer vescence. This air he called fixed air, because it exists fixed in their bodies. He inferred, that it pos sessed peculiar properties ; that it differed from corn. 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 chewed, 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 inferences 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 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 tv.-enty•five.