The fifth and sixth parts, lead the reader considerably further into a knowledge of the economy and practice of this art. They contain an account of a great variety of experiments which have been pursued on a large scale, in order to ascertain the most profitable mode of employing the retorts, the differences of opinion which have existed among practical men with respect to the degree of temperature fittest to be applied, and the number of hours at a time during which the retorts may most advantageously be kept in action, with the particular results which the experiments instituted into these points have afforded; and such other data, as will enable the reader to adopt that mode of operation, which under every circumstance of locality will be found most advantageous. The changes which have taken place with respect to the retorts, have been before detailed in part fourth; but in order to give the manufacturer a nearer insight into the superior advantages attending retorts of the construction lately brought into use, I have given in part seventh, a detailed description of the horizontal rotary retorts, the application of which has led to a more economical, expeditious, and easy method of manufacturing coal gas than heretofore practised. I have distinctly pointed out the advantages which these retorts present, the particular results they afford, and the method of applying them. The purification of coal gas forms the subject of part eighth. I have compared here, the apparatus for purifying coal gas, as it was originally constructed, with the improved machinery lately adopted, showing the inefficacy and defects of the former, and the decided superiority which belongs to the latter. The ninth part gives an account of the various improved gas holders which have been invented, and now are in action at the most recent establishments, for the purpose of storing large quantities of gas. The improvements that have been made in this department of the Gas Light machinery, are particularly valuable and have contributed more perhaps than any other, to lessen the expence of manufacturing gas for commercial purposes. In the tenth part, I have given a description of an entirely new machine, called the gas-metre, or self-acting guage, lately adopted at the Birmingham, Chester, and other gas works, which measures and registers the quantity of gas manufactured in any given time, from any given quantity of coal, or consumed during any period, by any number of burners or lamps. The great services which such a machine must render both to the manufacturer and consumer of gas, are particularly pointed out, and illustrated to the manufacturer, by serving as a complete check on his workmen as to the quantity of work that ought to be performed, and to the consumer, as an exact measure of the quantity of gas he receives, and ought to pay for.