cannot be affected by any opinion expressed by the authors. In the descriptions, which in the first instance have generally been furnished by the manufacturers of the apparatus, no attempt has therefore been made to appraise the particular generators, and comparisons and eulogistic comments have been excluded. The descriptions, however, would nevertheless have been somewhat out of place in the body of this book; they have therefore been relegated to a special Appendix. It has, of course, been impossible to include the generators of all even of the English manufacturers, and doubtless many trustworthy ones have remained unnoticed. Many firms also make other types of generators in addition to those described. It must not be assumed that because a particular make of generator is not mentioned it is necessarily faulty. The apparatus described may be regarded as typical or well known, and workable, but it is not by reason of its inclusion vouched for in any other respect by the authors. The Appendix is intended, not to bias or modify the judgment of the would-be purchaser of a generator, but merely to assist him in ascertaining what generators there are now on the market. The observations on the selection of a generator which follow, as well as any references in other chapters to the same matter, have been made without regard to particular apparatus of which a description may (or may not) appear in the Appendix. With this premise, it may be stated that the intending purchaser should regard the mechanism of a generator as shown in a sectional view or on inspection of the apparatus itself. If the generator is simple in construction, he should be able to understand its method of working at a glance, and by referring it to the type (_vide_ Chapter III.) to which it belongs, be able to appraise its utility from a chemical and physical aspect from what has already been said. If the generator is too complicated for ready understanding of its mode of working, it is not unlikely to prove too complicated to behave well in practice. Not less important than the mechanism of a generator is good construction from the mechanical point of view, _i.e._, whether stout metal has been employed, whether the seams and joints are well finished, and whether the whole apparatus has been built in the workman- like fashion which alone can give satisfaction in any kind of plant. Bearing these points in mind, the intending purchaser may find assistance in estimating the mechanical value of an apparatus by perusing the remainder of this chapter, which will be devoted to elaborating at length the so-called scientific principles underlying the construction of a satisfactory generator, and to giving information on the mechanical and practical points involved.