accumulation of the already much condensed, luminous matter may complete it in time. How far the light that is perpetually emitted from millions of suns may be concerned in this shining fluid, it might be presumptuous to attempt to determine; but, notwithstanding the inconceivable subtilty of the particles of light, when the number of the emitting bodies is almost infinitely great, and the time of the continual emission indefinitely long, the quantity of emitted particles may well become adequate to the constitution of a shining fluid, or luminous matter, provided a cause can be found that may retain them from flying off, or reunite them. But such a cause cannot be difficult to guess at, when we know that light is so easily reflected, refracted, inflected and deflected; and that, in the immense range of its course, it must pass through innumerable systems, where it cannot but frequently meet with many obstacles to its rectilinear progression not to mention the great counteraction of the united attractive force of whole sidereal systems, which must be continually exerting their power on the particles while they are endeavouring to fly off. However, we shall lay no stress on a surmise of this kind, as the means of verifying it are wanting; nor is it of any immediate consequence to us to know the origin of the luminous matter. Let it suffice, that its existence is rendered evident, by means of nebulous stars. FOOTNOTES: [Footnote 17: This excerpt and the one following are from the report by Herschel in the _Transactions of the Royal Society of London_; the third is an abstract from the same report, the conclusion, however, being by Herschel.] XVI KARL WILHELM SCHEELE 1742-1786 _Karl Wilhelm Scheele, who discovered independently of the English chemists the double constitution of air, was born in Stralsund, Pomerania, December 19, 1742. At an early age he manifested interest in pharmacy, and during his career as an apothecary engaged in various experiments in chemistry. He published his “Treatise on Air and Fire” in 1777. He died at Köping, May 21, 1786._ THE CONSTITUENTS OF AIR[18] 1. It is the object and chief business of chemistry to separate skilfully substances into their constituents, to discover their properties, and to compound them in different ways. How difficult it is, however, to carry out such operations with the greatest accuracy, can only be unknown to one who either has never undertaken this occupation, or at least has not done so with sufficient attention.