The discovery which Dr. Black had made that marble is a combination of lime and a peculiar substance, to which he gave the name of _fixed air_, began gradually to attract the attention of chemists in other parts of the world. It was natural in the first place to examine the nature and properties of this fixed air, and the circumstances under which it is generated. It may seem strange and unaccountable that Dr. Black did not enter with ardour into this new career which he had himself opened, and that he allowed others to reap the corn after having himself sown the grain. Yet he did take some steps towards ascertaining the properties of _fixed air_; though I am not certain what progress he made. He knew that a candle would not burn in it, and that it is destructive to life, when any living animal attempts to breathe it. He knew that it was formed in the lungs during the breathing of animals, and that it is generated during the fermentation of wine and beer. Whether he was aware that it possesses the properties of an acid I do not know; though with the knowledge which he possessed that it combines with alkalies and alkaline earths, and neutralizes them, or at least blunts and diminishes their alkaline properties, the conclusion that it partook of alkaline properties was scarcely avoidable. All these, and probably some other properties of _fixed air_ he was in the constant habit of stating in his lectures from the very commencement of his academical career; though, as he never published anything on the subject himself, it is not possible to know exactly how far his knowledge of the properties of _fixed air_ extended. The oldest manuscript copy of his lectures that I have seen was taken down in writing in the year 1773; and before that time Mr. Cavendish had published his paper on _fixed air_ and _hydrogen gas_, and had detailed the properties of each. It was impossible from the manuscript of Dr. Black’s lectures to know which of the properties of _fixed air_ stated by him were discovered by himself, and which were taken from Mr. Cavendish. This languor and listlessness, on the part of Dr. Black, is chiefly to be ascribed to the delicate state of his health, which precluded much exertion, and was particularly inconsistent with any attempt at putting his thoughts down upon paper. Hence, probably, that carelessness about posthumous fame, and that regardlessness of reputation, which, however it may be accounted for from bodily ailment, must still be considered as a blemish. How differently did Paschal act in a similar state of health! With what energy did he exert himself in spite of bodily ailment! But the tone of his mind was quite different from that of Dr. Black. Gentleness, diffidence, and perhaps even slowness of apprehension, were the characteristic features by which the latter was distinguished.