Neither is it likely that Scheele would have been acquainted with Mayow’s writings, though it is singular that he escaped the notice of Lavoisier who I believe was employed under government in the collection of essays on the theory and manufacture of saltpetre and in the superintendance of the saltpetre works, especially as Mayow was mentioned though disrespectfully by Lemery, in his paper on nitre before referred to. But there certainly is no evidence that Lavoisier obtained his ideas of oxygen and its combination with the blood from Mayow, or his theory of metallic calcination from Jean Rey, though his obligations to Dr. Priestley have not been always acknowledged with the candour and liberality that men of science would expect from Lavoisier. Mayow had more than ordinary discernment in comparing known facts, and drawing conclusions from them, but he does not appear to have had the talent of imagining decisive experiments, of varying them, of observing and noting all the natural phenomena attendant upon them, or sufficient industry in pursuing them. It is one thing to make a plausible conjecture, and another to verify it. Those alone are entitled to the honour of discoveries who not merely start the theory, but take the pains of pursuing it by experiments and resting it on the basis of well conceived and accurately ascertained facts, sufficiently numerous and varied to obviate the most prominent objections. Mayow has reasoned with great acuteness and conjectured with singular felicity, but he added little to the mass of philosophical KNOWLEDGE in his day. He composed and decomposed nitre and ascertained the existence of vital air in this substance as well as in the atmosphere, but he did not collect, exhibit, and examine it. He knew how to make artificial air from nitrous acid and iron, but all the extraordinary properties of this gas, remained unobserved by him as well as by others until collected and imprisoned by Dr. Priestley, and exposed to the question under his scrutinizing eye. Indeed as an experimentalist Dr. Priestley stands unrivalled. The multiplicity of his experiments, their ingenuity, their bearings upon the point in question, their general importance, and their fidelity, were never equalled upon the whole, before or since. Nor is it any detraction from their merit with those who are accustomed to experiment, that they hold out no pretensions to that suspicious accuracy, which has too often depended more upon arithmetical calculations than upon actual weight and measure. The many kinds of aeriform fluids discovered by Dr. Priestley, the many methods of procuring them, the skilfull investigation of their properties, the foundation he laid for the labours of others, the simplicity, the novelty, the neatness, and the cheapness of his apparatus, and his unequalled industry, have deservedly placed him at the head of pneumatic Chemistry. Nor should it be forgotten that while