It is, however, only within comparatively recent years that its true principles of construction have been at all fully recognized, as evinced by the burners which have been produced. For a long period, Argand burners were made upon wholly empirical and arbitrary rules. During the early years of gas lighting, the makers of gas apparatus, and such persons as professed to have a special knowledge of the production and utilization of the new illuminant, appear to have been ignorant of even the most obvious of the conditions required for the successful working of the burner. In one of the earliest works which appeared relating to gas lighting,[11] we find the Argand burner described as consisting of "two concentric tubes closed at the top with a ring having small perforations, out of which the gas can issue; thus forming small distinct streams of light." According to this description, the burner referred to cannot have been an Argand in the strictest sense of the word; but, in reality, must have consisted chiefly of a series of single jets placed in a circle, and surrounded by a glass chimney. But the great improvement in the amount of light developed, which resulted from bringing the jets of flame closer together, so as to cause them to coalesce and produce one homogeneous mass of flame, could not long escape notice; and accordingly we find that in "Clegg's Treatise," which appeared twenty-five years later, the proper disposition of the holes in the ring, necessary for the successful operation of the burner, is clearly recognized. In this work, speaking of the Argand burner, it is remarked (p. 193) that "the distance between the holes in the drilled ring should be so much that the jet of gas issuing from each shall, when ignited, just unite with its neighbour." [11] Accum's "Treatise on Gas-Lights."